add support for explicit event fetching from rpc

[faheelsattar]

Currently observer subscribes to smart contract events and waits for the events to be streamed towards it via websocket. This at certain time due to connection and ws issues causes for some events to be missed.
A much better approach would be to track synced until block for a certain event and explicitly query for events using that blocknumber.

events to update for

• transaction_submitted_event
• validator_registration_message