Switch to shared raft-boltdb library and add metrics

[raskchanky]

I didn't see any kind of run loop or built-in mechanism for executing tasks on a repeating interval so this is what I came up with. Is this the right approach?

[ncabatoff]

Side note: it looks like hashicorp/raft-boltdb#22 is going to be merged soon, and there is talk of us switching to using it and junking our local embedded copy of it, which you're modifying here. In which case maybe we should forget about using metricsutil as I was suggesting in my comment, since the way you're doing it now will lend itself better to being applied to raft-boltdb as a standalone library.