Simplify initialization of db connection and nullability in SqliteCache

[AntonyCorbett]

Avoid the null-forgiving operator for _db and Commands by initializing both in the ctor. No change in functionality here.

[mqudsi]

Hello Antony,

I've merged this w/ some changes to minimize diffs against master. I'll commit the nullability annotations separately.

Thanks for your contribution!

[mqudsi]

On second thought, SetBulkAsync() may handle null input but the API doesn't really want to be called with a null parameter and that would bind us to handling that in the future (whereas now we would be within our rights to throw an ArgumentNullException).