Drop support for streamable multibulk responses.

Supporting this feature has been problematic and leaded to some ugly
code to make abstractions such as pipelines and transactions aware of
these kind of response objects. Furthermore, it was not possible to
add them to all the connection classes due to implementation limits.

For such reasons Predis do not support them globally anymore, but the
actual classes are still shipped within the library so that they can
be used to build custom stuff at a level lower than client (that is,
unless we decide to remove them for good before going stable).
Merge branch 'v0.9/drop-multibulk-iterators'
Tweak phpdocs and API of streamable multibulk iterators.

Despite not being a globally supported feature of Predis anymore, they
are still optionally supported by our default text protocol processors
and they can be used to build custom stuff for specific needs.
Rework the API for external protocol processors.
Rationalize API renaming or moving stuff in Predis\Protocol namespace.
Update CHANGELOG.
Remove useless class constants.
Change visibility of some members.

There's no actual need for these ones to be private.
Minor tweaks to phpdoc descriptions.
Minor tweaks to code.

Also removed a useless method, most likely a leftover from the past.
Remove some obsoleted or useless methods from Predis\Client.
Slightly change the internal context factory method of Predis\Client.
Update CHANGELOG.
Rename Predis\Client::multiExec() to Predis\Client::transaction().

Method was deprecated since Predis v0.8.5.
Merge branch 'v0.9/protocol-processor-api' into integration