tstore: Get rid of all file system caches.

[lukebp]

The tstore backend and the plugins currently cache data using the politeiad file system. This was the quick and dirty way to get the tstore changes released, but its not a scalable approach and you run into cache incoherency issues since the cached data is not saved using an atomic operation. The tstore design has been finalized and all cached data should be moved to the key-value store.

The append only nature of tlog might make it difficult to run the entire record update, including caching required data, using a single atomic operation. We should investigate the best way to do this and document any limitations that we come across.