Add a CacheDirectory option

Tor puts a large number of large, rapidly changing documents in its DataDirectory, which defaults to /var/lib on many systems. This means that the size of backups blows out when running tor 0.3.1.1-alpha or later.

If we added a CacheDirectory option, it could default to /var/cache (or the appropriate paths on other operating systems), and be ignored by most backup tools.

This directory should contain

• cached directory documents, including cached (micro)descriptors and consensuses, and compressed directory document diffs
• any other files that tor can easily regenerate or re-download

This might also benefit mobile users with small disks, but application size appears to be less of a priority for mobile developers than CPU or network usage.

This is a breaking change for tools that expect to find consensuses in the data directory. Perhaps we should make CacheDirectory default to DataDirectory instead?

See this thread for background: https://lists.torproject.org/pipermail/tor-relays/2017-November/013542.html