Safer API

[FZambia]

Several changes to solve:

1. Disconnect and presence callback ordering
2. Deadlock when closing connection from SubRefresh callback
3. TransportClient to separate user-level type from transport-level client

Incompatible changes:
- (*Client).Close: changed from func(*Disconnect) error to func(*Disconnect) chan error
- (*Client).Handle: removed
- Config.ClientPresencePingInterval: removed
- NewClient: changed from func(context.Context, *Node, Transport) (*Client, error) to func(context.Context, *Node, Transport) (*TransportClient, CloseFunc, error)
- NodeEventHub.ClientRefresh: removed
- RefreshHandler: changed from func(context.Context, *Client, RefreshEvent) RefreshReply to func(RefreshEvent) RefreshReply
Compatible changes:
- (*ClientEventHub).Presence: added
- (*ClientEventHub).Refresh: added
- CloseFunc: added
- Config.ClientPresenceUpdateInterval: added
- ConnectReply.ClientSideRefresh: added
- PresenceEvent: added
- PresenceHandler: added
- PresenceReply: added
- TransportClient: added