Do not allocate space in cache-tier devs for MDA

[agrover]

We aren't saving state to cache-tier devs, so why allocate it?

We may wish to instead allocate reserved space, so that the actual used part of the cache dev still lands on a nice boundary like 1 MiB.

[mulkieran]

This strikes me as less urgent than a lot of things. There is no harm, just an excessive use of space.

And we should make certain to rule out a good reason to keep the BDA uniform before we start. I can't see any reason as of now, but there may be.

[agrover]

Definitely not worth doing/not gonna happen for 1.0. We may not want to do it ever, TBH. Is saving a few MiB on a cachedev really valuable these days?