MEDIUM: ring: protect the reader's positions against writers

MEDIUM: ring: protect the reader's positions against writers

The reader now needs to protect the positions it's reading. This is
already done via the readers counter at the beginning of messages,
but as long as the lock is present, this counter is decremented
before starting to parse messages, and incremented at the end.

We must now do that in reverse, first protect the end of the messages,
and only then remove ourselves from the already processed messages, so
that at no point could a writer pass over and possibly overwrite data
we're currently watching.