Force factory method usage for default implementations

[kzu]

Default implementations TableRepository<T> and TablePartition<T> can't really be used easily with DI, and having both direct constructor invocations and factory methods is confusing as to which one should be used when.

It's still useful to have the implementations available as public types, since someone might want to inherit them to provide more sensible defaults for the constructor arguments that aren't easily injectable (the Func<T, string> for partitionKey and rowKey).

So make the constructors protected internal instead.