Zero dynamic_buffer.size when it is cleared

To assist in parsing 'connections' stats, also reset dynamic_buffer.size to
zero when we reset the buffer pointer.

Change-Id: I1c336f1723d2d52ba184053d1068ad51417d0a0a
Reviewed-on: http://review.couchbase.org/40369
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Jim Walker <jim@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>