Asynchronous Database Access Methods

[groue]

This PR adds asynchronous database access methods on DatabaseQueue, Pool, and Snaphot:

protocol DatabaseReader {
    #if compiler(>=5.0)
    func asyncRead(_ block: @escaping (Result<Database, Error>) -> Void)
    #endif
}

protocol DatabaseWriter {
    #if compiler(>=5.0)
    func asyncWrite<T>(
        _ updates: @escaping (Database) throws -> T,
        completion: @escaping (Database, Result<T, Error>) -> Void)
    #endif

    func asyncWriteWithoutTransaction(_ updates: @escaping (Database) -> Void)
}

asyncWrite and asyncRead use the Result type from the Standard Library, and are only available with a Swift 5+ compiler (Xcode 10.2+).

I'm worried that asyncWriteWithoutTransaction is easier to use than the safer alternative asyncWrite. Let's hope the very long name sounds like a warning for the users.

Those methods will fuel RxGRDB and GRDBCombine.

[groue]

Yeah @sobri909, it took years of iterations, and the advent of the standard Result, before we could shape the asynchronous APIs 😅

[sobri909]

@groue I didn't even know this was in the works! Very cool surprise 😄

Also, apologies for being absent for the past many months. My workload has been insane, and drowned out my chances to work on anything else. But I do still read every GRDB email 😂 There's always fun new things turning up! I just never have the time to start using them or work on them. Sigh.

I'm keen to have a play with GRDBCombine though, once I get a chance. That looks extremely promising!

[groue]

Don't worry :-) I'm having a lot of fun, and it's a pleasure to share it 👍