v0.2.0 - improved service discovery

Mechanisms for service discovery:

• pull-based via ServiceDiscovery trait.
• API for push-based:
  • add a peer,
  • remove a peer,
  • set peers all at once.

Included simpler examples.

v0.1.0 - initial release

• port of golang's groupcache: https://github.com/golang/groupcache:

In comparison:

• does not implement groups (which makes me wonder whether groupcache name still makes sense).
  if such functionality is needed, it can be implemented on top of this library by having different implementations of ValueLoader depending on key prefix.
• allows to set TTL for cached values