spawn: use explicit strand executor

spawn: use explicit strand executor

the default spawn::yield_context uses the polymorphic boost::asio::executor
to support any executor type

rgw's beast frontend always uses the same executor type for these
coroutines, so we can use that type directly to avoid the overhead of
type erasure and virtual function calls

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