Don't find files in couchkvstore lazily

We no longer need to do this because we now have full control of
the engine and this code path is much more deterministic. This
also fixes a sporadic unit test failure that was cause by not
reading the database local docs before trying to flush a vbucket.

Change-Id: I5c4b0552f279b1e7d0a695071ae503f464891b32
Reviewed-on: http://review.couchbase.org/43118
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>