Miscellaneous Improvements

[CGA1123]

• Wrap the response body with a Rack::BodyProxy in order to handle the edge case of streaming bodies.
• Using thread-local vs fiber-local variables for the implementation of the store -- fiber local variables are not shared in Enumerator
• Alternative solutions to using MAX_STATEMENT_TIMEOUT for MySQL deadlines.
• Listen/select on the unicorn.socket or puma.socket for a client disconnect, and drop the deadline to 0 when that happens.