Async methods to execute IDbCommands with RdbmsProvider

RdbmsProvider should provide the following methods:

• ExecuteNonQueryAsync(IDbAsyncCommand command, CancellationToken cancellationToken)

• ExecuteReaderAsync(IDbAsyncCommand command, CancellationToken cancellationToken)

• ExecuteScalarAsync(IDbAsyncCommand command, CancellationToken cancellationToken)

Introduce new interface IDbAsyncCommand that emulates IDbCommand with async operations.

Note: it is not sensible to use DbCommand instead of IDbCommand in the API due to the abstract base type's resctrictions compared to the interface and it's API compatibility with the exisiting design.