Code review comment for lp:~kdub/mir/protobuf-exchange-buffer

(1) src/shared/symbols.map additions:
RAOF has pointed out to me today that function additions (which are not ABI breaking) need to be represented in new blocks in symbols.map. Because they do not form part of the current ABI, but are part of a future ABI. Confusingly, these new blocks can't be named according to the _next_ ABI either, for reasons RAOF just explained to me. So we need to come up with a naming scheme for new (temporary) symbols.map stanzas that exist until the next ABI break. On the next ABI break everything gets coalesced into a single block again.