MB-7701: latency optimized XDCR

In this commit, we optimistically send all docs to remote cluster
without sending a rev_diff beforehand. This optimization is meant
to improve the replication latency at the cost of potentially more
bandwidth usage in some use cases.

The feature is controlled by a new boolean ns_config parameter:

xdcr_latency_optimization

Like other xdcr parameters, users are able to override the ns_config
parameter by a boolean environment parameter:

XDCR_LATENCY_OPTIMIZATION

When a replication is created, this parameter will be read and stored
in the bucket replicator along with other replication options, and
applied to all vb replicators under that bucket replicator. Therefore
users need to stop/restart the replication to make it effective. It
does not make sense to apply this parameter to only a subse of vb
replicators during an ongoing XDCR.

Change-Id: Iee6428583728f0645fc3282163d892a6a6670e06