Add a Duration object and use it for TTL

[schwern]

Rather than taking a simple Integer number of microseconds, take a Duration object.

Ruby doesn't have a Duration object, and ActiveSupport::Duration is too big a dependency.

• Support accepting duration object(s) in addition to Integer milliseconds.
  • Find a widely supported Ruby duration object OR
    • https://www.rubydoc.info/gems/ruby-duration/ perhaps?
      • Accept it as is, but it only has second granularity.
      • Patch it to accept milliseconds instead of writing our own?
  • Write a really basic Momento::Duration object (or find a small gem).
    • Takes days, hours, minutes, seconds, milliseconds.
• Accept :max for maximum TTL?

Related to #35