[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_framework/util.py

@@ -249,6 +249,8 @@ def wait_for_bitcoind_start(process, datadir, i, rpchost=None):
                 raise
         time.sleep(0.25)
 
+def wait_for_node_exit(node_index, timeout):
+    bitcoind_processes[node_index].wait(timeout)
 
 def _start_node(i, dirname, extra_args=None, rpchost=None, timewait=None, binary=None, stderr=None):
     """Start a bitcoind and return RPC connection to it