the rpc transition will require sync and async objects to share a single
io buffer (since defining serializers on async is an immense pain in the
tail, slow, and ultimately not necessary). a generic buffer class allows
us to reuse existing serializers more readily (reuse them at all, even).
Change-Id: I5ebba8449f26f2bb76016818928183c7e0123be0
eldritch horrors
7d681a5049