Improve diagnostics provided by the driver (partial backport of #1472)

* Make connection pool check out failure exception more informative

* Distinguish monitoring connections from operation ones

* Increase default connection checkout timeout to 10 seconds

* Add connection pool summary for diagnostics

Author: p-mongo

lib/mongo/server/monitor/connection.rb

@@ -235,7 +235,7 @@ def handshake!(socket)
         end
 
         def retry_message
-          "Retrying ismaster on #{address}"
+          "Retrying ismaster in monitor for #{address}"
         end
       end
     end

spec/mongo/server/monitor/connection_spec.rb

@@ -155,7 +155,7 @@
 
       it 'logs the retry' do
         expect(Mongo::Logger.logger).to receive(:warn) do |msg|
-          expect(msg).to match(/Retrying ismaster on #{connection.address}/)
+          expect(msg).to match(/Retrying ismaster in monitor for #{connection.address}/)
         end
         expect(result).to be_a(Hash)
       end