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.
