Make Proxy.PassThrough async

[mostafa]

Currently the implementation of the gateway is synchronous and requires these steps:

1. Receive data (query) from the client.
2. Run onTrafficFromClient hook.
3. Send data (possibly modified query) to the server (PostgreSQL).
4. Run onTrafficToServer hook.
5. Receive data (PostgreSQL response) from the server.
6. Run onTrafficFromServer hook.
7. Send data (modified PostgreSQL response) to the client. This is done using async write.
8. Run onTrafficToClient hook.

This is serial, and should be fixed by using some sort of concurrent mechanism to wait for either server/client independently.

• Related to Handle bi-directional communication between server and client #32

[mostafa]

Fixed in #343.