MAB Distributed Persistence with Redis

[axsaucedo]

What this PR does / why we need it:

Initial exploration of distributed state persistence for online models. Implemented persistence for MAB TS usecase.

This PR extends the MAB case study to show how redis can be used to store and share state across multiple replicas (and across restarts).

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

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[seldondev]

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[seldondev]

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