MB-11927: Close backup database file

We should close the db file after no more data written.

Change-Id: I138c60f41746a503ba2688f732cd8af46aea0258
Reviewed-on: http://review.couchbase.org/41583
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>