MB-9376: Transmit the size of reduce function source

This way Couchstore can easily allocate enough
memory dynamically for reading the reduce function.

Change-Id: Ibe5095c219ca0a17467aa1a96ac58266bc186c5a
Reviewed-on: http://review.couchbase.org/29619
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>