add `BlockUntilServerActive`

[ZZiigguurraatt]

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.

[hieblmi]

Instead of polling GetState one could also subscribe to the lnrpc StateService to get notified once ready, see https://github.com/lightningnetwork/lnd/blob/23602e017e30c2f2810a17a62a174d8858b6077c/rpcperms/interceptor.go#L317

[ZZiigguurraatt]

Instead of polling GetState one could also subscribe to the lnrpc StateService to get notified once ready, see https://github.com/lightningnetwork/lnd/blob/23602e017e30c2f2810a17a62a174d8858b6077c/rpcperms/interceptor.go#L317

Good idea!

[ZZiigguurraatt]

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.

The reason we need this is because if we don't wait, we will get

rpc error: code = Unknown desc = lnd is not ready for: /lnrpc.Lightning/GetInfo