Assertion failed in fdb_kvs_header_copy inside fdb_compact

An fdb_compact operation is called in KV-engine to periodically compact the forestDB files. During one of the calls, an assertion is hit. Note that compaction thread inside KV-engine creates its own file handle.

The following 2 patches applied to ep-engine master and the rebalance_in_with_ops in simple-test should reproduce the problem.

http://review.couchbase.org/#/c/64934/
http://review.couchbase.org/#/c/65081/