Add v6 distribution client

[wagoodman]

Adds a new distribution.Client (for v6) responsible for:

• fetching a latest.json file
• determining if a given existing DB description is older or newer than a candidate update
• downloads candidate DB archives into a given directory, automatically failing if an expected archive checksum differs

One benefit of this approach is that the client is unaware of any installation concerns.

Partially implements #2125