Shut down the driver's connection pool safely (#94)

Shut down the EventLoopGroupConnectionPool using an API that doesn't call EventLoopFuture.wait() internally.

Sources/FluentSQLiteDriver/FluentSQLiteDriver.swift

@@ -27,7 +27,7 @@ struct FluentSQLiteDriver: DatabaseDriver {
     }
 
     func shutdown() {
-        self.pool.shutdown()
+        try? self.pool.syncShutdownGracefully()
     }
 }