CBSE-1407: Launch multi process for cbbackup/cbrestore

Add extra option parameter -x to cbbackupwrapper so that
it can pass it to cbbackup. Customers may want to tune
default values for parameter batch_max_size, batch_max_bytes
and recv_min_bytes to increase overall throughput.

Change-Id: I16a9e7582e04b9f67940f6202dc92ebf38ed6a3f
Reviewed-on: http://review.couchbase.org/43224
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>