MB-9638 UPR persistence sequence number command

This command is similar to the Tap checkpoint persistence command.
But we use sequence number instead of checkpoint id for UPR.
Refactored code to support both checkpoint id and seqno persistence.
The new memcached command opcode is CMD_SEQNO_PERSISTENCE 0xb7.
Added a stat "persistence_seqno".
Added a test case "upr_persistence_seqno".

Change-Id: Iec5db260e05cfa8889f253c33bb08209eb87ca2c
Reviewed-on: http://review.couchbase.org/30899
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>