rgw/main: add RGWProcessEnv in AppMain

rgw/main: add RGWProcessEnv in AppMain

instead of constructing a separate RGWProcessEnv for each frontend,
initialize a single instance in AppMain and share a reference with each
frontend

RGWRealmReloader now takes a reference to RGWProcessEnv instead of a
reference to the Store pointer itself, and updates RGWProcessEnv::store
on realm reload

because RGWRealmReloader may mutate RGWProcessEnv::store, we no longer have
a separate AppMain::store pointer, so don't worry about keeping that
consistent over reloads

Signed-off-by: Casey Bodley <cbodley@redhat.com>