Conn cleanup is async, plus other startup tweaks

- Connector reset, which happens when the connector is returned to the pool (DISCARD ALL), now happens
  asynchronously - the user thread returns immediately, before the operation completes. Note that the
  IsValid() ping, which happens before returning a connector to the user, is still there.
- Moved setting of ssl_renegotiation_limit to startup packet, was an additional roundtrip.
  This means that "DISCARD ALL" is sufficient to revert all params to their correct initial state.
- Cleaned up an old pre-protocol3 flow

Fixes #268 

@emill, @franciscojunior, can you please take a look?