btrace: resume all requested threads

btrace: resume all requested threads

The record targets are implicitly schedlocked.  They only step the current
thread and keep other threads where they are.

Change record btrace to step all requested threads in to_resume.

For maintenance and debugging, we keep the old behaviour when the target below
is not non-stop.  Enable with "maint set target-non-stop on".

gdb/
	* record-btrace.c (record_btrace_resume_thread): A move request
	overwrites a previous move request.
	(record_btrace_find_resume_thread): Removed.
	(record_btrace_resume): Resume all requested threads.