[#167] Requested reconnects should be immediate

This commit:
- Signf. simplifies the retry impln.
- Introduces the notion of retry-ids so that we can easily noop
  the effect of any pending retries just by switching out the active
  id.
- Simplified the client API: dropped `chsk-destroy!` and added
  `chsk-disconnect!`. The latter is just a reversible version of the
  latter.