Persist state

Feature Merge Request

Related Issue

closes #173 (closed)

Description of Feature

• assessments, concepts and the latest block will be stored (at most 1 per second)
• processed events also save their blocknumber
• fetchLatest assessments only goes back util the last saved block

next:

• save when an assessment was created, so that when one looks up the payouts/assessors one deos not have to give {fromBlock: 0} as a param

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