Set max retries to avoid queue congestion

When servers have *a lot* of remote slaves, some of them
unstable and potentially offline, a maximum retry policy is
needed to prevent push events to stay in the replication queue
and getting rescheduled forever.

Keep backward-compatible configuration by setting maxRetry
by default to zero, which means disabled.

Change-Id: I060cc7bc3a4d1089b0815db02d2e1430f83a2015