Architecture proposal documents

["Alan M. Carroll" <am...@thought-mesh.net>]

I have added a page for partial object caching - https://cwiki.apache.org/confluence/display/TS/Partial+Object+Caching
This will be changing because as I typed it up I realized I had a much better understanding of cache mechanics which is changing my view on the best options to do this.

I have also updated the tiered storage proposal, which is in the architectural docs historically and I don't have the energy to move it. At some point the update will appear here -
    https://trafficserver.readthedocs.org/en/latest/arch/cache/tier-storage.en.html
but you can grab it from git any time.