Added property CBLReplication.suspended to support apps that do background fetches

The app delegate’s -application:didReceiveRemoteNotification:fetchCompletionHandler: method
should set replication.suspended=NO, wait for the replication to download the new stuff, and
then set replication.suspended=YES before finishing.

NOTE: This is not part of the official cross-platform API, rather an
iOS-specific addition.

Fixes #339