Add config option to disable replication event emission

The plugin posts a stream event at every point of the replication
lifecycle (ref-replicated, replication-scheduled, replication-done,
project-deletion-*). When no listener is configured these events are
pure overhead. The cost is especially visible in multi-primary
deployments running an events-sharing plugin that fans every
replication event out to all the other primaries, even though nothing
on either end consumes it.

Introduce 'replication.emitEvents' (default true, so existing behavior
is preserved) to suppress emission at the source.

Change-Id: I32972d371ec83856e6783164bad9a229d1c8f934