CBD485-prereq: Add couchstore_set_purge_seq

In order to move the compactor into ep-engine for CBD-485
ep-engine needs to have a way to set the purge sequence number
Adding a new couchstore api routine
couchstore_set_purge_seq to set a sequence number into the header.

Change-Id: Id29f41d999a51604332fa0994a9be1c3b45c6a87
Reviewed-on: http://review.couchbase.org/29233
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>