Handle unlink/remove failures

The unlink/remove function can fail if there is another process that
has an open file handle on that file. In such cases, we need to retry
the unlink periodically in the flusher task until we get rid of those
files.

Change-Id: I4bfcf29b3fa866ec4946db658a245c722f3725ce
Reviewed-on: http://review.couchbase.org/41268
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>