Support asynchronous transactions

[simolus3]

Since Room uses an asynchronous-only connection pool, integrating it into the PowerSync SDK would be more efficient if we didn't have to call runBlocking around every statement.
Making query methods asynchronous also simplifies some interfaces, since it means that the top-level Queries and inner transactions can share asynchronous methods.

So, this PR:

1. Introduces the ScopedReadQueries and ScopedWriteQueries interfaces. The get, getAll and getOptional methods have been moved to ScopedReadQueries; execute is now part of ScopedWriteQueries.
2. The Queries interface now extends from ScopedWriteQueries.
3. Instead of implementing virtually every method twice (once in PowerSyncDatabaseImpl to await for initialization and then in InternalDatabaseImpl), most methods are now implemented directly in Queries ontop of the useConnection interface.

• This doesn't break waiting for initialization, as useConnection does that for all methods.

I have marked the synchronous callbacks as @Deprecated. I'm not sure if we want to be that aggressive or if we want to keep supporting both methods for longer. I'm slightly leaning toward eventually removing them given the nicer unified interface and reduced complexity.