fdb_close() called after fdb_get_file_info() throws an assert in ForestDB

KV-engine get periodic stats calls from ns_server at which point a new file handle is opened and a call to fdb_get_file_info is called. A subsequenct fdb_close() results in an assert.

The following 2 patches should help in reproducing the issue in KV-engine

http://review.couchbase.org/#/c/64934/ (Provide file fragmentation statistics to ns_server)
http://review.couchbase.org/#/c/65081/ (Sync writer tasks, open new handle for readers, reschedule delete vBuckets)