cli_wallet caching issues

_From @theoreticalbts on January 21, 2016 20:44_

The cache design of `cli_wallet` is fundamentally broken, leading to numerous issues: https://github.com/cryptonomex/graphene/issues/323 https://github.com/cryptonomex/graphene/issues/331 https://github.com/cryptonomex/graphene/issues/353 https://github.com/cryptonomex/graphene/issues/527

Anything which updates the account object causes a tendency for the cache to get out of sync. Really the CLI wallet should use the same architecture as the web wallet: Subscribe to object updates in order to implement a caching proxy for all object requests from individual wallet functions. As a stop-gap we could simply remove the existing cache, implement a new cache and require it to hit the `witness_node` anytime it needs an account (assuming most `cli_wallet` users also run `witness_node` on localhost or a fast LAN).

As this requires a substantial re-working of the wallet, perhaps this should be a worker proposal.

_Copied from original issue: cryptonomex/graphene#530_

Tasks to be done:
- [x] internal get_account function
- [ ] list_my_accounts
- [ ] account_id change when there is a fork (is this a separate issue?)