* remove some superfluous gcsafes
* remove getTailState (unused)
* don't store old epochrefs in blocks
* document attestation pool a bit
* remove `pcs =` cruft from log
per spec, we must half-close request stream - not doing so may lead to
failure of the other end to start processing our request leading to
timeouts.
In particular, this fixes many sync problems that have been seen on
medalla.
* remove safeClose - close no longer raises
* use per-chunk timeouts in request processing
removed in 0.12.2 - the flow, in particular when the other peer doesn't
support snappy, is hard to follow because of the trial-and-error
approach - removing it simplifies things and removes some of the
hard-to-read parts of the thunking etc
