- Eliminate a couple of ancient TODOs. Extend the test.
- Fix `ConnectionPool::send` docstring appropriately.
- Tweak timeouts in test because it used to fail on my machine.

NOTE: there's still a bug, because we set each new replicaset's weight
to 1 (even ones which don't satisfy replication factor)
before the bucket distribution is bootstrapped. But there must be at
least one replicaset with non zero weight in order for vshard.*.cfg to
work.

A potential solution would be to only configure vshard once a replicaset
is filled up.

Async callbacks support is very poor in rust, so using a macro is one of
the few ways to have somewhat friendly code.

The code inside process_topology_request_async has become too complex
therefore TopologyRequest has lost its utility

refactor: rename UpdateInstanceRequest -> update_instance::Request, UpdateInstanceResponse -> update_instance::Response

Use direct structure names instead of aliases for better IDE code surfing