Shut down the driver's connection pool safely (#220)

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

Sources/FluentPostgresDriver/FluentPostgresDriver.swift

@@ -25,6 +25,6 @@ struct _FluentPostgresDriver<E: PostgresJSONEncoder, D: PostgresJSONDecoder>: Da
     }
 
     func shutdown() {
-        self.pool.shutdown()
+        try? self.pool.syncShutdownGracefully()
     }
 }