* Fix missing database shutdown on WalletAppConfig.stop()
* Cleanup BitcoindBlockPollingTest
* Cleanup ProcessBlockTest and all tests that use CachedBitcoind
* Cleanup BitcoindBackendTest
* Cache rescan thread pool rather than creating a new one everytime
* Refactor AsyncUtil to just use an execution context rather than an actor system
* Create async util project, use it in various projects that depend on it