Improve irdest-core storage API

Currently the services section of the irdest-core API has three main endpoints that services can use to store data.

• save(UserAuth, Service, MetadataMap, TagSet)
• delete(UserAuth, Service, Key(String))
• query(UserAuth, Service, TagSet)

The MetadataMap type is defined as follows:

pub struct MetadataMap {
    name: String,
    map: BTreeMap<String, Vec<u8>>,
}

The problem with this approach is that diff-operations need to be handled by the service, and this logic can not be re-used by other services.

The solution

• Remove save function endpoint
• Create an insert function endpoint with the following properties
  • Each service has access to one store map (the MetadataMap may be re-usable)
  • Function API takes (UserAuth, Service, Key, Value)
  • Key is a (String, String) tuple, that acts as (Namespace, Key)
  • Value is bound as V: Serialize to be used by the service as it sees fit
• The delete and query endpoints can be re-used, albeit their function APIs need to be adjusted.
  • query will now only return a single T, bound T: DeserializeOwned.

Some future considerations to make the general storage subsystem in irdest-core simpler is to approach the alexandria API differently. Notes towards this should be tracked in #4.