MB-9730 Add support for auto-purge during modify_btree

When `modify_btree` is invoked with a list of add/remove operations,
tree node purging can be performed along with the operations applied
on the btree. Tree purging can be enabled by setting the flag,
rq.enable_purging = true and specifying rq.purge_kp and rq.purge_kv
functions.

Added unit test for auto-purge as well.

Change-Id: I2566b1947128ea7830fb61ddb7260bbb798c1e1f
Reviewed-on: http://review.couchbase.org/30446
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>