RUST-1608 Clean shutdown for Client

[abr-egn]

RUST-1608

This adds the shutdown and shutdown_immediate methods on Client. Terminating the workers was straightforward but closing outstanding resources (cursors and sessions) was very much not. Initially, I was hoping that shutdown could actively track and clean up those when called, but the shared ownership required for that would have required API changes that were both backwards-incompatible and generally much less easy to work with.

What I've ended up with is using the pre-created tasks we discussed in RUST-1439; while this doesn't mean they're guaranteed to run when the runtime shuts down, it does mean that the Client can track and await them. Concretely, this allows shutdown to wait for all pending resources to have been dropped and complete async cleanup. This also fixes RUST-1099.

This solution does have a couple of downsides:

• It pushes part of the "orderly shutdown" responsibility on to the user, requiring that all resources be dropped before shutdown will return. For normal usage, this shouldn't be an issue - Rust's typical scoped memory management should mean that most of the time that happens automatically, and when it doesn't it should be easy to fix. In case that's not possible, shutdown_immediate provides an escape hatch that does just the worker thread termination without the resource wait.
• There's a potential race condition where the user could create a cursor or session after shutdown has waited for resources but before it closes out the workers. I think this mostly falls into the "please don't do that" sort of misbehavior.

[isabelatkinson]

code changes look good, although it looks like there are some lint/test failures in CI

[abr-egn]

code changes look good, although it looks like there are some lint/test failures in CI

Ah, yep, fixed the lint, and I'd forgotten to restrict the transaction-using tests to transaction-supporting servers. The cargo-deny failure is unrelated (see RUST-1674 for details).

[abr-egn]

Updated with changes to unify the handling for shutdown and sync_worker. Beyond removing some duplication, this will also be very useful for implementing the "fill pool to min" method.

[isabelatkinson]

lgtm! nice work with this