Introduce Redis abstractions for an in-memory datastore

[wkerckho]

Similar to #22, services use the Redis API directly, as opposed to interactions with other dependencies such as MongoDB, Clickhouse and Gubernator, which are encapsulated in generic interfaces. We can improve the quality of the codebase, by introducing an abstraction layer for the functionality that is currently provided by Redis directly. From what I see now, this is mostly restricted to various flavors of inter-process shared data structures. For example: a key-value store with TTLs for representing expiring team or data set invitations, or a set of active streaming sessions, etc.

Ideally, this abstraction layer should also be usable as a scalable cache layer for the NGSI-LD implementatin.

[wkerckho]

Note that in essence, Redisson, the library we are using, is an abstraction on top of Redis. Maybe we can also achieve what we need by having some pre-configured Obelisk data structures (built upon Redisson) made available via CDI?

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