Server timeouts in the NIO implementation

[glbrntt]

At the moment the NIO implementation has timeouts on the client but not on the server. We should add timeouts to the server as well.

[MrMage]

Adding context from https://github.com/grpc/grpc-swift/pull/459#discussion_r282356734:

When adding that, we should also make sure that all promises (status and response) are being fulfilled (probably with an error) when the timeout occurs.

It might also be nice to set a maximum timeout on the server object, to prevent malicious clients from opening many requests with very long timeouts.

[SebastianThiebaud]

This issue can probably be resolved once https://github.com/grpc/grpc-swift/pull/850 is in.

[glbrntt]

This is a different issue: it's related to timing out an RPC based on the grpc-timeout header sent by the client.