Import following downloaded blocks if the preceding block is already queued

Before this commit, following blocks were dropped and this behavior
was causing "parent not found" errors in Block Sync.

Author: majecty

sync/src/block/extension.rs

@@ -728,6 +728,9 @@ impl Extension {
                     Err(BlockImportError::Import(ImportError::AlreadyInChain)) => {
                         cwarn!(SYNC, "Downloaded already existing block({})", hash)
                     }
+                    Err(BlockImportError::Import(ImportError::AlreadyQueued)) => {
+                        cwarn!(SYNC, "Downloaded already queued in the verification queue({})", hash)
+                    }
                     Err(err) => {
                         // FIXME: handle import errors
                         cwarn!(SYNC, "Cannot import block({}): {:?}", hash, err);