Move batch features into main classes and clean up

The classes I'd created originally to implement getting/setting in
batches each included some logic that relied on what Redis type was
being used. I realized it made more sense just to move all of that into
the _RedisTypes and then implement a batch mode for RedisObject, instead
of having a separate RedisObjectStream class.

Essentially, now you can supply 'batch_size' and 'transaction_size'
values (optionally) when instantiating a RedisObject, and this triggers
batch mode -- get and set operations work in batch (a la
RedisObjectStream). When the 'defer' option is also set to True, it
overrides any value provided for 'transaction_size' and forces commands
not to be executed.

For _RedisTypes, I added a new _BatchMixin class for implementing the
needed batch methods. This let me create a separate parallel chain of
inheritance via _SequenceBatchMixin for the different rtypes that are
sequence-like (list, zset) or partially sequence-like (hash, set,
string).

I also did some miscellaneous cleanup here.
- Moved the option to execute set/get commands into _RedisType. This
  keeps all of that behavior consistent, since batch_save and batch_get
  must be able to trigger pipe.execute (for executing transactions).
- Added tests and concrete behavior for edge cases. Index lookup values
  that are out of range. Errors for trying to batch get/set rtypes that
  do not support that (sets, encoded_objs).
- Updated docstrings based on recent changes.

Author: jthomale