Implement a simple data store

[localvar]

Components like filters and controllers often need persistent data and share the data inside the cluster, we already have the Custom Data for this but it is too complex to use in most cases. Another way is to use the functions of the cluster package directly, but it is dangerous because it can access any data of the cluster, a wrong implemented component can easily break the whole cluster.

So we'd like to implement a simple data store that meets the below requirements:

1. the store has its own fixed (hard-coded) key prefix
2. when creating an instance of the store, the user needs to provide a second-level prefix.
3. the store instances provide functions like the cluster package, for example: Get/Set/GetPrefix/SetPrefix and etc.
4. the actual key used to access data is the two levels of prefixes and the provided key.
5. client APIs to manage(create/update/delete/list) the data, and the egctl command need to be updated accordingly.

[suchen-sci]

Custom data is added now.