Expose whether the SQLiteConnectionWithLock is locked

I want to enforce the `SQLiteConnectionWithLock` to be locked before calling any other methods on it, such as `Table<T>()`.

Given a wrapper class around `SQLiteConnectionWithLock`, like so:

```
public class Repository<T>
{
	protected readonly SQLiteConnectionWithLock connection;

	public BaseRepository(SQLiteConnectionWithLock connection)
	{
		this.connection = connection;
		connection.CreateTable<T>();
	}

	public IEnumerable<T> All()
	{
		AssertConenctionLocked();
		return connection.Table<T>();
	}

	public IDisposable Lock()
	{
		return connection.Lock();
	}

	private void AssertConenctionLocked()
	{
		if (!connection.IsLocked()) throw new NotLockedException();
	}
}
```

This allows me to enforce the call to `repository.Lock()` before `repository.All()`, so the following code will fail:

```
var connection = GetConnection();
var repository = new Repository<T>(connection);
repository.All();
```

but this won't:

```
var connection = GetConnection();
var repository = new Repository<T>(connection);

using (repository.Lock()) {
	repository.All();
}
```