rgw: async_defer_chain() that detects transition to cls_rgw_gc

rgw: async_defer_chain() that detects transition to cls_rgw_gc

this replaces the defer_chain() function with optional asynchrony with
one that's only async. in the pre-transition case, it arranges for a
librados callback to detect the transition and retry against cls_rgw_gc

the RGWRados::gc_aio_operate() interface now requires the caller to pass
in an AioCompletion to support async_defer_chain()'s custom callback

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