Package store implements a shared store backed by a kvstore or similar with
the following properties:

* A single type is used to represent all keys
* Any number of collaborators can join the store. Typically a collaborator
  is an individual Cilium agent running on each node.
* All collaborators can own and contribute keys to the store. Each key is
  owned by exactly one collaborator. It is the responsibility of each
  collaborator to pick a key name which is guaranteed to be unique.
* All collaborate desire to see all keys within the scope of a store. The
  scope of the store is defined by a common key prefix. For this purpose,
  each collaborator maintains a local cache of all keys in the store by
  subscribing to change events.

This implementation will be used by the upcoming kvstore based node discovery

Signed-off-by: Thomas Graf <thomas@cilium.io>