Add the accounts service to status manager. This will allow us to query
the status of the accounts service and see if it is running or not.
For incoming gRPC requests to the accounts service, we also use the
status manager to check if the accounts service is running or not to
determine if we should let the request through or not.
Pass the status manager to the sub-server manager so that the sub-server
manager can inform the status manager of any sub-servers that have
started or failed to start.
Modify the sub-server Manager's AddServer method to take an `enabled`
boolean so that it can handle what to do with a disabled sub-server.
This will come into play in a future commit which adds a status server.
Add a new `WhiteListedURLs` method to the `SubServer` interface
so that Lit can easily collect the set of permissions from each
sub-server that does not require a macaroon.
Due to the rpcProxy being started early now, it could be the case that a
call is made to `GetRemoteConn` before the remote connection has
actually been set up. This commit catches this case so that an error can
be returned and a panic avoided.
