mds: make MDS.replay_done clearer

... and add some assertions.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: refactor MDS boot

* Make boot_start private.
* Define boot stages in enum, replace int with type.
* Merge steps 0 and 1, 0 always fell through to 1.
* Replace switch() with breaks with series of if()

Signed-off-by: John Spray <john.spray@redhat.com>