FileStore: apply changes after disabling m_filestore_replica_fadvise

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit ed8b0e65bde14d0a3a08bc233dee6a997e379dcc)
FileStore: handle observers in constructor/destructor

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Merge remote-tracking branch 'upstream/next'