Enable Tor to work without using filesystem for cached files

Looking at mid-term improvements for Tor integration within third party application, this ticket is to enable Tor to work without ever touching the filesystem, but keeping basic caching functionalities.

Today Tor need to write the following files: cached-certs
cached-consensus
cached-microdesc-consensus cached-microdescs cached-microdescs.new cached-descriptors cached-descriptors.new state lock

Those files could be removed/disabled as a first step, but as far as i understood, it would bring some performance impact.

It would be nice if a third party application:

before shutdown could download via Tor CP a copy of that data structures
immediately after startup could upload via Tor CP of the most recent cached data structures

This ticket, when bundled with legacy/trac#6411 (moved) and legacy/trac#13865 (moved), would allow a completely filesystem-free use a Tor, being entirely governed by a third party app (e.g,: using TxTorCon or Orbot as controllers).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information