[agw] Remove inheritance relationship between task state managers and common state managers

[ardzoht]

Description

• MME, S1AP, SPGW and NGAP tasks are using the StateManager singleton class which wraps functionality to allocate the UE and task state, as well as reading and writing the state back and forth into the redis database.
• This was introduced to create a single point of state allocation for any task that holds state in the MME.
• This creates an unnecessary complex dependencies between the StateManager, StateConverter and tangles the responsibilities of reading/writing state to redis as well as allocating the local state in the hashtable.

Goal

• The goal is to move away from the StateManager singleton and create different classes for the tasks to allocate the UE and task state there, and to potentially do the reads/writes to redis in a separate class.

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[ssanadhya]

@rsarwad , moving our Slack conversation here. Your question on this issue is:
" Across all tasks, there are some common functions like, get_imsi_str(), is_persist_state_enabled(), clear_ue_state_db() and some variables like table_key, task_name, is_initialized, persist_state_enabled etc. Can we keep these common parameters in base class and definition also remains same across all tasks?"

Yes, that should be okay. The goal of this issue is to separate the responsibility for Redis read/write to each task, so that each task can write its own state without being blocked on other tasks. @ardzoht , what do you think?

[ardzoht]

Agree @ssanadhya, these function should be common across mme tasks, so the definition should not change across them.

[rsarwad]

Thanks @ssanadhya and @ardzoht for clarification