Merge lp://qastaging/~mir-team/mir/fix-1465883-and-1341490 into lp://qastaging/mir

Bump the libmirprotobuf ABI from 0 to 1. Technically we've broken the
ABI plenty of times without bumping. But it becomes a real problem in
the wild when we do a release that also bumps the client ABI.

If you have a newer libmirprotobuf0 used by both an older libmirclient8
and newer libmirclient9 then only libmirclient9 will know the correct
object sizes -- objects that we instantiate on the stack. Existing
clients in that case that still use libmirclient8 will then be
instantiating message objects with the wrong size/structure on the
stack which leads to stack corruption and client crashes.
(LP: #1465883) (LP: #1341490)

And we do wish to keep instantiating message objects on the stack
because that helps with IPC/RPC performance not touching the heap
on every message.

Also keep in mind mirprotobuf is a "private" ABI. We are free to bump
it without needing to bump the public ABIs like libmirclient. Because
clients have no direct dynamic linkage to anything in libmirprotobuf.
.