This was meant to be part of the previous stack of commits, but I pushed the
wrong stack. This patch addresses a number of code review feedback points, the
most visible being to remain 'contains' to something else (in this case
'getmissing').
The old way of fetching from the server required the base store api expose a way
for outside callers to add fetch handlers to the store. This exposed some of the
underlying details of how data is fetched in an unnecessary way and added an
awkward subscription api.
Let's just treat our remote caches as another store we can fetch from, and
require that the over arching configure logic (in shallowrepo.py) can connect
all our stores together in a union store.
The last major piece of functionality that needs to be moved into the new store
is the gc algorithm. This is just a copy paste of the one that exists in
localcache.
