Add functionality for Postgres Listen and Notify

[JonEustace]

Postgres offers a pub / sub listen and notify, it would be amazing if Vapor 3 could implement this. Perfect for realtime updates.

[pedantix]

I can dig in on this. assuming just like redis then we should be able to mimick command names in appliance

Subscribe

Info (https://www.postgresql.org/docs/10/static/sql-listen.html)

conn.listen(_  channel: String,   @escaping (PostgreSQLData) -> Void) -> Futue<Conn>

Publish

Info (https://www.postgresql.org/docs/10/static/sql-notify.html)

conn.notify(_ channel: String, payload: PostgrSQLData) -> Futue<Void>

[pedantix]

In Progress no one complained so I am implementing

[tanner0101]

Publicized in #72:

let conn = try PostgreSQLConnection.makeTest(transport: .cleartext)
let done = conn.listen("foo") { message in
    XCTAssertEqual(message, "hi")
    return true
}
do {
    let conn = try PostgreSQLConnection.makeTest(transport: .cleartext)
    _ = try conn.notify("foo", message: "hi").wait()
}
try done.wait()