MB-8422: enable XDCR parameters to immediately effective

Today several key XDCR parameters are read once when replication
starts from ns_config or env parameters. These values will be used
all through XDCR, even users change the parameter in the middle of
replication. To make new parameter effective, users need to stop
and restart replication, which is expensive since new replication
will have to start from the last persisted checkpoint or the very
beginning to scan all the data that have been replicated. This is
also inconvenient for users including QE and performance team to
test and tune system performance by using different parameters.

This commit will make updating following XDC parameters effective
immediately:

- xdcr_optimistic_replication_threshold
- xdcr_worker_batch_size
- xdcr_connection_timeout
- xdcr_num_worker_process
- xdcr_num_http_connections
- xdcr_num_retries_per_request

The changed parameter  will be dumped to log and to avoid query the
ns_config too often, the new value will be read and effective when
the next vb replicator is scheduled.

Change-Id: I75bbc8f3c9a017b4218cf3aec8fd89d2586d652a
Reviewed-on: http://review.couchbase.org/26837
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>