rgw: refactor rgw_main

rgw: refactor rgw_main

This change is an extended version of work by Orit Wasserman
moving some request-processing class headers and definition to
their own files (RGWProcess, RGWProcessEnv, RGWProcessFrontend),
along with process_request.

I have taken the refactoring further, so as to remove all class
declarations and member definitions from rgw_main.cc, so that the
file retains just:

* the responsibility to parse arguments and supervise the daemon
* the responsibility to select from front-ends/configure the RGW service
* usage

I have made small adjustments:
* moving some processing out of headers, to remove circular deps
* removed unused headers from rgw_main.cc
* added editor boilerplate and tweaked include guards in some files

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>