NIO client declares itself as `.ready` too soon

[glbrntt]

Issue Description

Issue #517 indicated that we can have an infinite reconnection loop if the connection closes immediately after the channel becomes active. We should transition the connection state to .ready after we have received e.g. the initial settings frame on the connection.

[MrMage]

Should be fixed in https://github.com/grpc/grpc-swift/releases/tag/1.0.0-alpha.2.