introduced xdc_lhttpc_pool for all our lhttpc connections

Previously every single instance of couch_api_wrap database we've used
was creating it's own separate http connections pool.

It can be easily seen how it causes tons of sockets in TIMED_WAIT
state. I.e. due to connections frequently opened/closed.

xdc_lhttpc_pool is a central pool of connections that solves
TIMED_WAIT problem. And moves us closer towards supporting
xdcr-over-ssl proxy.

Change-Id: If554588220cad208054ccd07341398428e12f017
Reviewed-on: http://review.couchbase.org/30347
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>