MB-11642: Allow changes to tap/dcp batch size

Based on the type of connection we want to be able to change the
batch size in order to prioritize one connection over the other.

Change-Id: I39789eb63743ee7b3735df1a61f5289e9ddd321b
Reviewed-on: http://review.couchbase.org/41757
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>