Fix a bug that takes dirty seq number during snapshot open
Fix a bug that takes dirty seq number during snapshot open

- Even when block reuse is enabled, we should ignore dirty seq number
when there are newly allocated blocks since the last committed header.

Change-Id: Iab0fe57fa701372142d1fdf7bf3abda4c81c48c1

wurikiji/forestdb,couchbase/forestdb,hisundar/forestdb,couchbase/forestdb,hisundar/forestdb,wurikiji/forestdb