Breaking Change: Redlock now consistently uses seconds for all time units

In particular:

1. The auto_release_time argument to Redlock.__init__() and synchronize() is now in seconds (was previously milliseconds)
2. Redlock.locked() now returns how much longer you'll hold the lease in seconds (or 0 if you don't hold the lock; and was also previously milliseconds)

What's Changed

• Remove superfluous return statement by @brainix in #626
• In Redlock, consistently measure time in seconds by @brainix in #627

Full Changelog: v2.3.7...v3.0.0