Refactored frozen constants in connection, gazelle and request into
const.
Also renamed some of those to match Puma whenever possible - const's
base is taken from Puma.

There are a few constants that are commented out, I left them there
because we may need them at some point and I think it's handy to have a
reference ready to go.
Merge pull request #12 from cotag/refactor-consts

Refactored frozen constants into const