Asynchronous database access

[groue]

This PR brings support for reactive asynchronous reads and writes:

let dbQueue: DatabaseQueue = ...

// Async write
let write: Completable = dbQueue.rx.write { db in
    try Player(...).insert(db)
}

let newPlayerCount: Single<Int> = dbQueue.rx.writeAndReturn { db in
    try Player(...).insert(db)
    return try Player.fetchCount(db)
}

// Async read
let players: Single<[Player]> = dbQueue.rx.read { db in
    try Player.fetchAll(db)
}

Advanced scheduling for DatabasePool:

// Async write + read
let newPlayerCount: Single<Int> = dbPool.rx.write(
    updates: { db in try Player(...).insert(db) },
    thenRead: { db in try Player.fetchCount(db) })

Breaking changes

The observation methods have been renamed from fetch... to observe...:

-Player.all().rx.fetchOne(in: dbQueue)
-Player.all().rx.fetchAll(in: dbQueue)
-Player.all().rx.fetchCount(in: dbQueue)
+Player.all().rx.observeFirst(in: dbQueue)
+Player.all().rx.observeAll(in: dbQueue)
+Player.all().rx.observeCount(in: dbQueue)

The way to provide a specific scheduler to a value observable has changed:

-Player.all().rx.fetchAll(in: dbQueue, scheduler: MainScheduler.asyncInstance)
+Player.all().rx.observeAll(in: dbQueue, observeOn: MainScheduler.asyncInstance)

[groue]

• rename writeCompletable to write, and write to writeAndReturn (like catchErrorJustReturn)