MB-9552: reduce message passing in XMem

Today in XMem XDCR, the vb replicator rep_worker process need to
send all docs to replicate to xmem_server process first, and the
xmem_server process will pass all these docs to its child xmem_worker
process. Consequently there are two big message passing among Erlang
processes which may cause some inefficiency.

This commit will make rep_worker process send all docs to replicate
directly to xmem_worker process and flush to remote. Therefore it
removes the first message passing above and initial XDCR test on
toybuild shows more than 15% throughput improvement.

Change-Id: Iccf3ab02137b30d9f9bdd5c41c6d168ec7f93da0