tombstoned: refactor request dequeuing a bit.

Also make it loop, so that upon failing to start a dequeued crash
request, we continue to the next one.

Bug: http://b/36685795
Test: debuggerd_test
Change-Id: I94889125f16f4681c6fa0fa9cac456302602ce01
(cherry picked from commit 807a45807b679ced93ca3af971a30874cffd1929)
1 file changed