[tlog] Remove all local storage caches

[marcopeereboom]

#1180 adds tlog functionality to replace the no longer viable git backend. This PR unfortunately uses the local filesystem to generate various caches. These caches can easily become stale or corrupt and are inherently non-scalable. What needs to happen is that all local caches need to be killed and replaced with a global memory cache of sorts. Either redis or memcached are exactly meant for this use case.

the tlog backend should manage the cache which means it should inherently be coherent so long additions and removals are called in the proper order. Thus every call should be made directly to tlog and let tlog deal with caching. The pseudo code looks like this:

func GetTlogValue(x) y {
  y, err := getFromCache(x)
  if err != nil {
    y, err = getFromTlog(x)
    if err != nil {
      return err
    }
    insertCache(x, y)
  }
  return y
}

func DelValue(x) error {
  delFromCache(x)
  return delFromWherever(x)
}

[lukebp]

Whoops, I added a duplicate of this in #1391. Closing this one since we've decided to handle this another way and the above comment no longer applies.