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.
Expand acceptable negotiated ciphers

Both AES256 and AES128 are fine. The purpose of this test was to avoid
3DES in particular.
Make lzo compression in the pipeline optional
README typo fixes
Add Sean Linsley <xovatdev@gmail.com> as a contributor
Use s3-external-1 endpoint for us-standard

This is the endpoint in us-east-1, where most instances are.  This is
not being applied to the v0.6 maintenance releases to avoid changing
behavior for those who are closer to Seattle.
Elide other backends besides S3 in requirements.txt

It's not clear how to make things cleanly optional but not a usability
nightmare (e.g. installing *no* backends by default for 'pip install'
based people) within the Python packaging framework, so don't try..for
now.
Avoid deleting keys that have a new-ish timestamp

This is a paranoid safeguard against mess-ups; even if turned against
the wrong prefix it is likely many backups will be retained.

Some of the tests need adjustmenmts to not freak out about this
safeguard.
Increase socket timeout to deal with pathological cross-region issues
Try to fix timeouts by using cat semi-async-i/o exclusively
Implement manifests and checksumming of backups

After a download, one can run now run "wal-e backup-verify" to make
sure that all files are present and, optionally, have expected
content.

Slightly editorialized by Daniel Farina <daniel@heroku.com>