This commit improves the `coder server` shutdown procedure so that all
triggers for shutdown do so in a graceful way without skipping any
steps.

We also improve cancellation and shutdown of services by ensuring
resources are cleaned up at the end.

Notable changes:
- We wrap `cmd.Context()` to allow us to control cancellation better
- We attempt graceful shutdown of the http server (`server.Shutdown`)
  because it's less abrupt (compared to `shutdownConns`)
- All exit paths share the same shutdown procedure (except for early
  exit)
- `provisionerd`s are now shutdown concurrently instead of one at a
  time, the also now get a new context for shutdown because
  `cmd.Context()` may be cancelled
- Resources created by `newProvisionerDaemon` are cleaned up
- Lifecycle `Executor` exits its goroutine on context cancellation

Fixes #3245

* fix: Improve `coder server` shutdown procedure

This commit improves the `coder server` shutdown procedure so that all
triggers for shutdown do so in a graceful way without skipping any
steps.

We also improve cancellation and shutdown of services by ensuring
resources are cleaned up at the end.

Notable changes:
- We wrap `cmd.Context()` to allow us to control cancellation better
- We attempt graceful shutdown of the http server (`server.Shutdown`)
  because it's less abrupt (compared to `shutdownConns`)
- All exit paths share the same shutdown procedure (except for early
  exit)
- `provisionerd`s are now shutdown concurrently instead of one at a
  time, the also now get a new context for shutdown because
  `cmd.Context()` may be cancelled
- Resources created by `newProvisionerDaemon` are cleaned up
- Lifecycle `Executor` exits its goroutine on context cancellation

Fixes #3245