A thread could terminate between the point when we stop for a
breakpoint and the point where we send a stop signal to all threads, if
this happens setCurrentBreakpoints will fail with an error.
We should tolerate this.
For some reason this happens very frequently when running delve on
processes with the race detector enabed.
RequestManualStop will run concurrently with trapWait, since one writes
dbp.halt and the other reads it dbp.halt should be protected by a
mutex.
Updates #830
While implementing the gdbserial backend everything was changed to call
Detach to "close" a process so that gdbserial could do its clean up in
a single place. However the native implementation of Detach does not
actually kill processes we launched.
Fixes#821
