Be independent on system time

Time may differ on instances running, so the lock acquiring timeout may be different than what's configured if two systems sharing the lock has time drifting away from each other.

What about writing something "random" (collision resistant; just incremental?) rather than `:created` time (https://github.com/clearhaus/dynamodb-mutex/blob/master/lib/dynamodb_mutex/lock.rb#L36) and then each instance checks if it has seen the given "random" for `:ttl` time?

Then `:ttl` becomes kinda "`:min_ttl`", or the TTL that will always be experienced. Needs to be documented. It's also easy to document that you need time on instances to be right, but the bugs you may experience with time drifting is probable less fun than the configuration trouble you have when setting the TTL.

Start by reading lib/dynamodb_mutex/lock.rb at the :created timestamp reference and trace how :ttl is used when acquiring and checking a lock. Define how instances should recognize a lock without relying on synchronized system time, then document the resulting :ttl or :min_ttl behavior. The issue does not mention tests to run.

Written by the indexing model from the issue text.