Refactor indexer in preparation for integration with Search indexer

[franckc]

Checklist:

• [x] Code contains relevant tests for the problem you are solving
• [x] Ensure all new and existing tests pass
• [x] Update any relevant READMEs and docs
• [x] Submit to the develop branch instead of master

Description:

This diff refactors the current indexer logic to make it easier to add new Indexers (ex: Search) in the future. The new flow is as follow:

• The event_listener() function which is periodically scheduled every N seconds via Celery and fetchs events from the Ethereum network now calls an EventHandler object for processing events.
• Given an event to process, the EventHandler inspects it to determine its type and calls the appropriate methods from the indexers/notifier.

In subsequent diffs I'll implement the SearchIndexer class and configure a search engine back-end (most likely ElasticSearch since it is supported off-the-shelve by Heroku).

[Gzing]

Will do a quick test on my local and approve but looks really good :+1: