Fix problems in exception dumper

Fixes:

* Hard C-Crash:  We currently have a case where we can crash inside the crash reporter, which means you get half-written crash dumps.  `dfw1-maas-prod-dcass0` was doing this and generated 23737 crash dumps before someone noticed.

* Reduce Concurrency in Crash Reporter:  We would currently try to unload every crash report in parallel... which is bad if you have 23,000+ of them.  Now limit it to 4 at a time.