identity service: complete rework of the caching

[adecaro]

Currently, the caching used by the identity service seems to be more problematic for performance.
This Issue asks to rework the entire strategy by first identifying which parts require which type of caching.

[adecaro]

This task is in connection to hyperledger-labs/fabric-smart-client#984 as well.
Banchmarking the identity service's sub-components will allow us to understand how we can improve this service.