Bug#556610: Please do incremental checks every night instead of a full monthly one

[Michael Tokarev]

Ok. I reviewed the patches and proposed solutions, but
I can't commit/implement any of them so far.

The main issue which all proposed solutions share is when
there's a large array, say, md0, and a small array, say,
md1, both shares the same set of underlying disks, so md
subystem will not check/repair them in parallel.  In this
situation, we will never check md1 if checking md0 takes
more time than we allow in a month (28 days).

I'll think about it all more.

Thanks,

/mjt