Backport temporary copies from GRDBSnapshotTesting

[groue]

This pull requests adds two convenience methods that create temporary copies of databases:

let path = "/path/to/database.sqlite"
let dbQueue = try DatabaseQueue.inMemoryCopy(fromPath: path)
let dbQueue = try DatabaseQueue.temporaryCopy(fromPath: path)

Those have originally lived in GRDBSnapshotTesting, but people who want to test databases without GRDBSnapshotTesting might like those methods. Their implementation is more robust as well, with better support for the readonly configuration flag.