[qa] Test non-atomic chainstate writes

Adds new functional test, dbcrash.py, which uses -dbcrashratio to exercise the
logic for recovering from a crash during chainstate flush.

dbcrash.py is added to the extended tests, as it may take ~10 minutes to run

Use _Exit() instead of exit() for crash simulation

This eliminates stderr output such as:
    terminate called without an active exception
or
    Assertion failed: (!pthread_mutex_destroy(&m)), function ~recursive_mutex, file /usr/local/include/boost/thread/pthread/recursive_mutex.hpp, line 104.

Eliminating the stderr output on crash simulation allows testing with
test_runner.py, which reports a test as failed if stderr is produced.

test/functional/test_runner.py

@@ -124,6 +124,7 @@ EXTENDED_SCRIPTS = [
     # vv Tests less than 5m vv
     'maxuploadtarget.py',
     'mempool_packages.py',
+    'dbcrash.py',
     # vv Tests less than 2m vv
     'bip68-sequence.py',
     'getblocktemplate_longpoll.py',