MB-7786 Group concurrent remote bucket requests.

When XDCR replication is created, every vbucket replicator calls
remote_cluster_info:get_remote_bucket_by_ref about the same time. At
this only one remote node is known. Namely, the one that was used to
create remote cluster reference. Previously remote_clusters_info did
not attempt to group all these concurrent calls into a single request
to the remote node. Which could lead to the overload on the
destination node. This would produce an error. Replicators would
retry and most likely hit cached value. So replication would start
without any issues. But the error is reported to user and creates
certain confusion.

To lower the probability of overloading any of destination nodes,
remote_clusters_info will not make more than one outstanding request
per node per remote bucket. So for the case described earlier, only
one remote request will be performed for all the simultaneous
get_remote_bucket calls.

Change-Id: I9ca0bb0dba5670c545f824d00ebaff9cf331ac06
Reviewed-on: http://review.couchbase.org/24986
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>