Add a wrapper around NSMutableDictionary for thread safety.
Add a wrapper around NSMutableDictionary for thread safety.

Where multiple threads can access the same NSMutableDictionary, these
have been wrapped in a new ThreadSafeMutableDictionary that uses a
serial dispatch queue to ensure that access to the underlying
NSMutableDictionary can only happen from one thread at a time.
ThreadSafeMutableDictionary is a minimal wrapper, wrapping only
the operations required by CDTSyncPlugin.m

cloudant/sync-cordova-plugin,cloudant/sync-cordova-plugin,cloudant/sync-cordova-plugin,cloudant/sync-cordova-plugin,cloudant/sync-cordova-plugin