Implement SE Account updating

[sappenin]

Once the Settlement RFC is finalized, update the Connector's AccountSettings object and Admin API controller to allow updates from the Connector Account to percolate to the settlement engine account, if necessary.

Currently, updating a Connector account doesn't do anything with the SE, so the SettlementEngineClient.updateAccount method is likely not necessary. Ditto for the client's delete account.

However, we should rethink this just to be safe, once the Spec has been finalized.

[sappenin]

Superseded by #220