add `BlockUntilServerActive`

We need to add `BlockUntilServerActive` to retry and wait until the node actually returns `SERVER_ACTIVE` from `GetState` before allowing the client to actually try and use lnd.

We also need to mention that doing so makes lndclient incompatible with lnd `v0.13` which doesn't support that.

@djkazic already has a plan to implement this fix, but creating an issue to track his resolution of it.

Start by tracing the client’s existing retry and GetState handling. Implement BlockUntilServerActive so use of lnd waits until GetState returns SERVER_ACTIVE, then verify the issue’s compatibility requirement is documented for lnd versions before v0.14. Done means the wait behavior is covered and the lnd v0.13 incompatibility is clearly mentioned.

Written by the indexing model from the issue text.