Create a generic Cache for other RPCs

[elliottwilliams]

Other RPCs could benefit from cache: agency.* could remember lists of routes and stations before they're fetched (which is a very expensive call). Yet-to-be-implemented Schedule RPCs could retain schedule information without having to fetch.

This issue extends the infrastructure used by LastEventCache into a generic Cache with the following methods:

• Cache.lookup(rpc:args:kwargs:) -> TopicEvent?
  Selects a cache for this particular rpc, and returns a stored TopicEvent if one exists matching the call argument.
• Cache.store(rpc:args:kwargs:event:) -> Bool
  Selects a cache for rpc, and attempts to persist `event.
• Cache.void(rpc:event:)
  Voids a stored event in the cache for rpc.

RPC-specific caches will implement a protocol with corresponding methods, as well as a test to determine if they accept a particular RPC. They can then be wired into a Connection.call middleware stack which tries each cache in order before trying the WAMP connection.