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ARROW-15732: [C++] Do not use any CPU threads in execution plan when use_threads is false (#15104)

This PR gets rid of the "executor==nullptr" style of execution from Acero.  Now a CPU executor must always be defined.  This also updates the DeclarationToXyz methods to support 5 "standard" modes of operation:

* sync / threaded - All CPU work is done on the CPU thread pool.  The calling thread sleeps until the plan is done.
* sync / serial - All CPU work is done on the calling thread.  The calling thread becomes a serial executor until plan is done.
* async / threaded - All CPU work is done on the CPU thread pool.  The calling thread returns immediately.
* async / serial - All CPU work is done on a plan-specific 1-thread pool.  The calling thread returns immediately.
* async / custom exec context - All CPU work is done on the provided executor which cannot be nullptr and the caller is responsible for keeping it alive.

For backwards compatibility the "old style" of serial execution (which is still used by R/python) will still be accepted.  Under the hood a 1-thread pool will be created and its lifetime will be bound to the exec plan (this is similar to async / serial when using DeclarationToXyz).

Since DeclarationToXyz doesn't support "ordered sinks" we cannot yet migrate the bindings and so they will continue to rely on the deprecated API.

All unit tests are updated to use the new API.  Some unit tests were not actually running in parallel (even though they said they were) and now that they are running in parallel a few changes had to be made to accept / handle non-deterministic result order.

As part of this process some unit tests were migrated away from StartAndCollect in favor of DeclarationToXyz.  Hopefully we can eventually migrate almost all of the unit tests to DeclarationToXyz but that is a longer term effort.

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>