Add information of origin to MemoryLimiterWorker

[posidron]

At the moment there is no information about where or what triggered a MEMORY_LIMIT_EXCEEDED which makes filing reports for the IPC fuzzers not really possible for those cases.

Possible idea:

def get_gdb_stacktrace(pid):
  return subprocess.check_output(["gdb", "--batch", "-q", 
                                  "-ex", "thread apply all bt full", 
                                  "-ex", "quit", 
                                  "-p", "%d" % pid])

[tysmith]

Added in commit 8d43d82 & commit 3e2f738. Thanks for the suggestion.