use thread pool in SyncClient to fix syncing issue
use thread pool in SyncClient to fix syncing issue

Using a separate thread for each game was causing syncing issues.
The suspected reason is that the garbage collector was destroying
threads because they were no longer accessible. Switch to using
a thead pool as the thread pool keeps a reference to each NetworkClient.

jdm64/GenesisChess-Android,jdm64/GenesisChess-Android