Should `close()` wait for the server to shutdown?

Lifting this from a PR comment so it isn't lost now that the PR is merged without addressing this:

@smklein
After https://github.com/oxidecomputer/dropshot/pull/537, I wonder if we should no longer make close wait for the server to terminate.

We could add a convenience close_and_wait for anyone that wants backwards compatibility?

@jgallagher
Changing the behavior of close() seems a little dicey from a backwards compatibility standpoint, but I do see your point. A couple other ideas, although I'm not sure I like these all that much more:

1. Deprecate close() and add two new methods: close_and_wait() + close_nowait().
2. Change close() to take an argument specifying whether or not to wait: close(CloseBehavior::Wait) or somesuch. This would be backwards incompatible in a way that old clients would have to update instead of silently getting different behavior.

Start by reviewing the close() API and the behavior changes discussed in PRs 537 and 543. Determine whether close() should wait, and define a backwards-compatible API such as close_and_wait() or close_nowait(); done requires an agreed behavior and interface.