Enable async modelset + refactor of model creation

[alonre24]

• This PR touches many source files. It is not final yet (looks like there are some problems with RDB loading + valgrind check is required)
• There are 2 main changed (both concern model set):

1. MODELSET command is processed in a background thread. The design is generic, so that this background thread will be able to get more types of "Jobs" in the future.
2. Refactor in creation of RAI_Model - instead of creating it for every backend separably, it is now created beforehand with all the common fields that are agnostic to the BE, and then every backend enriches the object fields (model/session) as needed.

TODO

• Replace RedisModule_Replicateverbatim with RedisModule_Replicate (in reply callback) - replicas currently not working for this command.
• Update the changes in RDB load v0
• Enable running the commad from within LUA/MULTI (without blocking).

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