[Notifications] Implement mechanism to prevent sending duplicates

[franckc]

For context, see #852
Since the event-listener does not guarantee only-once delivery, the notifications server should implement a mechanism to keep track of notifications already sent in order to avoid sending duplicates. Suggestion is to use Redis for storing that data.

[franckc]

@gauravsavanur07 Did you send an email saying you would be interested in working on this ? For some reason it did not get recorded in this issue...

Anyway, if you did, here are a few pointers/ideas, in terms of how I could imagine this would work to get you started:

• Define a scheme to create uid for messages received by notifications servers from the event-listener via webhook. The tuple (blockNumber, logIndex) could be a good choice.
• When receiving a new message, notifications server first checks against persistent storage if message has already been processed by doing a lookup by uid. If already processed, do nothing, otherwise send notification(s) to user(s). Then store uid in storage.
• For persistent storage, Redis is probably a good choice. We haven't set it up in production yet but will do soon since we need it for another service.

Let me know any questions...

[micahalcorn]

Considering the first version of this to be done ✅