librgw: re-derive RGWLibRequest

librgw: re-derive RGWLibRequest

Derive RGWLibRequest from RGWRequest, RGWHandler.

This strategy defines the dialect of ops following the derivation
pattern for RGWLib actions to be their handler.

On the negative side, RGWLibRequest has struct req_state* s from two
ancestors.  Since we don't use virtal inheritance, the only overhead is
the extra copy, and need for disambiguation in the direct descendants
(this is only 1 method in 1 file, atm).

The motivation is as follows.  RGWHandler currently has 3 responsibilities:
1) op-chasing; 2) to hold authorize() and read_permissions() methods
3) to initialize req_state

RGWLib actions have no use for the op-chasing behavior, and authorize()
and read_permissions() methods are arguably more naturally expressed as
methods on the actions.  Meanwhile struct req_state appears to be a
candidate for refactoring into the RGWRequest class hierarchy, which would
mean that initalization probably should move from hanlder to request, as
well.  Refactoring req_state is would be quite intrusive, so this change
defers that for later, but keeps the 1-1 mappings from request to
handler at 0 cost.

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