Session improvements

Session improvements

[José Valim]

Guys,

I'm trying to make Rails 3 and Rack share more code. While we are
already using a lot of middleware that ship with Rack, there are a few
more we could. That said, I decided to tackle the session middleware.
My first commit simply added a small feature and refactored/improved
the current code:

* Provide :cookie_only as option and true as default. If false, allows
SID to be retrieved from GET/POST params;

* Do not send the cookie back to the client if session id did not
change (improves both server and client side perf);

* Make Abstract::ID implementation more modular, allowing Cookie
implementation to be simpler by inheriting from it;

It can be found here:

http://github.com/josevalim/rack/commit/5dac45a38c38003f6608ca94fdf40d1d6f4e68ab

Later then, I provided another commit, also refactoring the current
Memcached and Pool stores. Basically, there was a bit of code
duplication and both stores were responsible to handle options[:drop]
and options[:renew]. That said, I decided to move the handling of such
options to Abstract::ID. The result can be found at:

http://github.com/josevalim/rack/commit/102a08f831cda86fbdcbe7cc98f5cc71ea19e496

Any feedback is welcome. I've also opened an issue in Github:
http://github.com/rack/rack/issues#issue/39

Cheers!