Add restore_checkpoint in raw restore stats
Add restore_checkpoint in raw restore stats

The `restore_checkpoint` stats shows the highest checkpoint id
of the first restored backup file by starting membase in
restore mode. `restore_checkpoint` indicates the checkpoint to
which the membase server is restored.

Change-Id: I0738a76eabce957513b37802cccf7c4b6c92251a
Reviewed-on: http://review.couchbase.org/12503
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

jimwwalker/ep-engine,couchbase/ep-engine,daverigby/kv_engine,sriganes/ep-engine,jimwwalker/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,abhinavdangeti/ep-engine,zbase/ep-engine,couchbaselabs/ep-engine,couchbase/ep-engine,hisundar/ep-engine,abhinavdangeti/ep-engine,sriganes/ep-engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/kv_engine,couchbaselabs/ep-engine,zbase/ep-engine,daverigby/kv_engine,membase/ep-engine,abhinavdangeti/ep-engine,couchbaselabs/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,sriganes/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,zbase/ep-engine,zbase/ep-engine,membase/ep-engine,jimwwalker/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,membase/ep-engine,owendCB/ep-engine,zbase/ep-engine,daverigby/ep-engine,couchbase/ep-engine,abhinavdangeti/ep-engine,couchbase/ep-engine,membase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,daverigby/ep-engine,couchbaselabs/ep-engine