gRPC: make gRPC calls using the C++ implementation:

* make `FSTRemoteStore` create C++ streams instead of their Objective-C
  counterparts;
* port firebase/firebase-js-sdk#1041: streams
  are never recreated, only restarted.
* make `FSTDatastore` delegate server calls to the C++ implementation.

Author: var-const

Firestore/Example/Tests/Integration/FSTDatastoreTests.mm

@@ -52,6 +52,7 @@
 using firebase::firestore::model::DocumentKeySet;
 using firebase::firestore::model::Precondition;
 using firebase::firestore::model::TargetId;
+using firebase::firestore::remote::GrpcConnection;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -162,7 +163,7 @@ - (void)setUp {
   NSString *projectID = [FSTIntegrationTestCase projectID];
   FIRFirestoreSettings *settings = [FSTIntegrationTestCase settings];
   if (!settings.sslEnabled) {
-    [GRPCCall useInsecureConnectionsForHost:settings.host];
+    GrpcConnection::UseInsecureChannel(util::MakeString(settings.host));
   }
 
   DatabaseId database_id(util::MakeString(projectID), DatabaseId::kDefault);

Firestore/Example/Tests/Util/FSTIntegrationTestCase.mm

@@ -98,10 +98,6 @@ - (void)tearDown {
       [self shutdownFirestore:firestore];
     }
   } @finally {
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wdeprecated-declarations"
-    [GRPCCall closeOpenConnections];
-#pragma clang diagnostic pop
     _firestores = nil;
     [super tearDown];
   }
@@ -165,9 +161,9 @@ + (void)setUpDefaults {
          "Alternatively, if you're a Googler with a Hexa preproduction environment, run "
          "setup_integration_tests.py to properly configure testing SSL certificates.");
   }
-  [GRPCCall useTestCertsPath:certsPath testName:@"test_cert_2" forHost:defaultSettings.host];
-  GrpcConnection::UseTestCertificate([certsPath cStringUsingEncoding:NSASCIIStringEncoding],
-                                     "test_cert_2");
+  GrpcConnection::UseTestCertificate(
+      util::MakeString(defaultSettings.host),
+      util::MakeString(certsPath), "test_cert_2");
 }
 
 + (NSString *)projectID {

Firestore/Source/Remote/FSTDatastore.h

@@ -16,14 +16,19 @@
 
 #import <Foundation/Foundation.h>
 
+#include <memory>
 #include <vector>
 
 #import "Firestore/Source/Core/FSTTypes.h"
 
+#include "Firestore/core/src/firebase/firestore//remote/watch_stream.h"
+#include "Firestore/core/src/firebase/firestore//remote/write_stream.h"
 #include "Firestore/core/src/firebase/firestore/auth/credentials_provider.h"
 #include "Firestore/core/src/firebase/firestore/core/database_info.h"
 #include "Firestore/core/src/firebase/firestore/model/database_id.h"
 #include "Firestore/core/src/firebase/firestore/model/document_key.h"
+#include "Firestore/core/src/firebase/firestore/remote/datastore.h"
+#include "absl/memory/memory.h"
 #include "absl/strings/string_view.h"
 
 @class FSTDispatchQueue;
@@ -32,9 +37,6 @@
 @class FSTQueryData;
 @class FSTSerializerBeta;
 @class FSTWatchChange;
-@class FSTWatchStream;
-@class FSTWriteStream;
-@class GRPCCall;
 @class GRXWriter;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -67,12 +69,7 @@ NS_ASSUME_NONNULL_BEGIN
                                          credentials  // no passing ownership
     NS_DESIGNATED_INITIALIZER;
 
-/**
- * Takes a dictionary of (HTTP) response headers and returns the set of whitelisted headers
- * (for logging purposes).
- */
-+ (NSDictionary<NSString *, NSString *> *)extractWhiteListedHeaders:
-    (NSDictionary<NSString *, NSString *> *)header;
+- (void)shutdown;
 
 /** Converts the error to a FIRFirestoreErrorDomain error. */
 + (NSError *)firestoreErrorForError:(NSError *)error;
@@ -83,11 +80,6 @@ NS_ASSUME_NONNULL_BEGIN
 /** Returns YES if the given error indicates the RPC associated with it may not be retried. */
 + (BOOL)isPermanentWriteError:(NSError *)error;
 
-/** Adds headers to the RPC including any OAuth access token if provided .*/
-+ (void)prepareHeadersForRPC:(GRPCCall *)rpc
-                  databaseID:(const firebase::firestore::model::DatabaseId *)databaseID
-                       token:(const absl::string_view)token;
-
 /** Looks up a list of documents in datastore. */
 - (void)lookupDocuments:(const std::vector<firebase::firestore::model::DocumentKey> &)keys
              completion:(FSTVoidMaybeDocumentArrayErrorBlock)completion;
@@ -97,10 +89,12 @@ NS_ASSUME_NONNULL_BEGIN
              completion:(FSTVoidErrorBlock)completion;
 
 /** Creates a new watch stream. */
-- (FSTWatchStream *)createWatchStream;
+- (std::shared_ptr<firebase::firestore::remote::WatchStream>)createWatchStreamWithDelegate:
+    (id<FSTWatchStreamDelegate>)delegate;
 
 /** Creates a new write stream. */
-- (FSTWriteStream *)createWriteStream;
+- (std::shared_ptr<firebase::firestore::remote::WriteStream>)createWriteStreamWithDelegate:
+    (id<FSTWriteStreamDelegate>)delegate;
 
 /** The name of the database and the backend. */
 // Does not own this DatabaseInfo.