Add missing license headers.
`NoStore`: Expose `MaxChannels`.
Clean up Emcee adapter configuration.

* Reintroduce ElastiCache support.
* Change `{Recv, Send, Poll, Retry, Handle}Timeout` configuration
  fields to strings. Reintroduce `parseDuration()` as `parseTimeout()`
  for converting timeout field values to `time.Duration`s.
Register Emcee as `"memcache"`.