[gdb/dap] Join JSON writer thread with DAP thread

[gdb/dap] Join JSON writer thread with DAP thread

The DAP interpreter runs in its own thread, and starts a few threads:
- the JSON reader thread,
- the JSON writer thread, and
- the inferior output reader thread.

As part of the DAP shutdown, both the JSON reader thread and the JSON writer
thread, as well as the DAP main thread run to exit, but these exits are not
ordered in any way.

Wait in the main DAP thread for the exit of the JSON writer thread.

This makes sure that the responses are flushed to the DAP client before DAP
shutdown.

An earlier version of this patch used Queue.task_done() to accomplish the
same, but that didn't guarantee writing the "<thread name>: terminating"
log entry from thread_wrapper before DAP shutdown.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR dap/31380
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31380