Avoid repeatedly loading account details from Redis

[emschwartz]

Even though Redis is a fast store, we should avoid loading and parsing the Account details every time we handle a request. Especially if we need a copy of all of them in-memory for the routing table, we should use an in-memory cache of account details and clone the Account object instead of re-loading the details from Redis every time we need them.

[gakonst]

On that note, we could also split the Account in smaller structs and/or make IldcpAccount part of the Account trait

[gakonst]

Should we implement an in-memory cache for any of the store's loaded accounts? (keyed by accountId) @emschwartz

[emschwartz]

I think that sounds like a reasonable idea