os/FileStore: fix journal-less operation

A few different fixes here:

- make the op_seq correct on start when there is no replay
- make an explicit 'no journal' case in queue_transaction (one that
  actually works).  Note that the 'trailing journal' mode is probably
  still broken.
- add a -J flag to vstart.sh for journalless operation

Signed-off-by: Sage Weil <sage@inktank.com>
os/FileStore: refactor backend instantiation

Use a factory method for creating the backend so that the logic is not
duplicated in mkfs() and _detect_fs().  Make the other fs-dependent logic
(xattr limits) key off of the fs_type magic.

Signed-off-by: Sage Weil <sage@inktank.com>
os/FileStore: include filestore backend, f_type in osd metadata

Report the FileStore backend and underlying fs type as part of the osd
metadata shared with the monitor.

Signed-off-by: Sage Weil <sage@inktank.com>
os: ObjectStore::collect_metadata unit tests

And an emacs-executable reminder about how to run a specific test.

Signed-off-by: Loic Dachary <loic@dachary.org>
os: FileStore::create_backend unit tests

Assert that the fs_type is mapped to a backend that claims to have the
matching name.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1864 from dachary/wip-filestore

unit tests for #1860
Merge pull request #1860 from ceph/wip-filestore

osd: filestore backend improvements

Reviewed-by: Loic Dachary <loic@dachary.org>