MB-9951: Trim rev-id to 16 bytes

With adding the datatype and flexible metadata to the the
metadata field [1], it is no longer fixed to 16 bytes, but
is at least 2 bytes bigger, one for the flex meta code and
one for the datatype field.

Hence trim the revision ID down to a maximum of 16 bytes.

[1] http://review.couchbase.org/31046

Change-Id: I25898dc4e70c1d7d071c228e2bcdd2c03e06ff6a
Reviewed-on: http://review.couchbase.org/32743
Tested-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>