The md_recoveryd thread is responsible for initiating and cleaning
up resync threads.
This job can be equally well done by the per-array threads
for those arrays which might need it.

So the mdrecoveryd thread is gone and the core code that
it ran is now run by raid5d, raid1d or multipathd.

We add an MD_RECOVERY_NEEDED flag so those daemon don't have
to bother trying to lock the md array unless it is likely
that something needs to be done.

Also modify the names of all threads to have the number of
md device.