Remove redundant conditional
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.
Add manifest files with checksums listing each file in each tar partition
first attempt at downloading separate manifests from s3 keys
Use separate S3 keys, add tests, reverse sense of --no-verify-checksums to --verify-checksums
Add documentation for new flags and command
By popular demand, move manifests to .wal-e/manifests and just keep the most recent restore info