politeiad: Implement fscks.

[lukebp]

One of the issues that was punted on in #1180 was implementing the filesystem checks that also would rebuild the caches. I was going to include this in #1406, but 1406 is taking longer then expected and this issue is blocking #1477. Let's go ahead and implement these separately so that we can get 1477 in and the legacy proposals migrated to the tstore backend on production.

I added the required fsck code in #1512 and notes on how to implement the cache rebuilds. Implement everything that is currently blocking 1477. Open PRs for the work in chunks. One PR for every cache that is being rebuilt.