Refactor: Use cookie.sendResponse in get_errmap_executor
Refactor: Use cookie.sendResponse in get_errmap_executor

sendResponse tries to insert the packet into the already
existing write buffer and only reallocate if needed,
whereas the old method would _always_ allocate a new
send buffer.

Change-Id: Ib417183b9ec7a48360724b2723a11f135fa7bf3e
Reviewed-on: http://review.couchbase.org/85742
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine