Track all ParameterStatus messages in the client
Add "ShortByteString0"

Right now, all ByteStrings and ByteString0s that come out of a FromBackend
message are slices into a single backing ByteString.  I think we'll
want to detach these for storage (e.g., parameter names and values)
and might as well move them out of pinned memory at the same time.
Request cancellation
More haddock

Also comments about why the "clear t" in makeSSL is important and
why receiveMessage/consume/consume' do not try to tolerate async
exceptions.