Fix channel shim test for gevent 1.0.0

Gevent 1.0 specifies this as its version, not 1.0.0, breaking the
comparison spuriously if one has version 1.0 installed exactly.
Implement parallelized, pipelined prefetching of WAL

This is intended to achieve a drastic performance increase in WAL
application in many situations.

This is done by using some temporary directories and daemonized
sub-processes, implement a prefetcher that can hopefully pipeline
downloading WAL to be applied while Postgres is performing other WAL
recovery.