WebSockets follow up

[ebuchman]

Websocket client was upgraded per #516. Lots of discussion there about minor follow ups and design improvements. Let's do the followup and capture the rest in the ADR:

• Test it with transactions / ensure its working for the ibc relayer
• Simplify/reduce number of features - rpc: Simplify client interface #569
• Send pings (keepalive) - rpc: event subscription management for RPC client #516 (comment)
• Update ADR:
  • Add concurrency diagram to clarify current concurrency model and how the components communicate with one another
  • Clarify the pending states - rpc: event subscription management for RPC client #516 (comment)
  • Add notes about possible alternative/future approaches to concurrency model eg.
    • Dropping the extra terminate channel and just using the cmd channel to communicate unsubsrciptions - rpc: event subscription management for RPC client #516 (comment) - Implemented in rpc: Simplify client interface #569!
• Possibly eliminate async/await from the core, use a synchronous websocket with read/write routines, and wrap it in an async API for consumers that need it - rpc: event subscription management for RPC client #516 (comment)

[ebuchman]

ADR updates addressed in #576 but see #576 (comment) re the need for pending.

[ebuchman]

Closing for #590 and #582