MB-9339: changed menelaus_rest to use lhttpc

We'll need this in order to move remote_cluster_info facility to xdcr
connection pool.

Change-Id: I337864646dfbab897967b90f0816fcc265491de5
MB-9339: use xdcr lhttpc pool for remote_clusters_info requests

Change-Id: Ibf607fd4131546e604148b27e8b9b08589746c5a
MB-9339: amputate xmem checkpointing mode

because it will not be supported by xdcr-over-ssl

Change-Id: I915f9a4eec835bd9032da05f589bec5ca072e7fb
MB-9339: support success in mc_client_binary:map_status

Change-Id: I82b064ee33b1630db43a95b4d23f34b14bf8c8af
MB-9339: introduced ns_connection_pool

Change-Id: I8e509a6459a88f536a087b86296c1d09f4eee37b
MB-9339: added pooled_memcached_client and memcached_clients_pool

Change-Id: I64473163306b12fb9a1b5ef1fba33d841801da86
MB-9339: use pooled_memcached_client for xmem

Change-Id: I79c5341188b06050f1a8033508fc0296d4c93421