Update our testnet indexer to support restarting from the last processed blockheight

[charleslavon]

Is your feature request related to a problem? Please describe.
The primary indexer used by test.near.org, discovery-stage-indexer is currently configured to support indexing blocks starting from a terraform configured START_BLOCK_HEIGHT. When something occurs which requires the indexer to be restarted, indexing starts from this start block height, or requires manual intervention to determine what was the last block processed and an update to terraform to set this as the START_BLOCK_HEIGHT.

Describe the solution you'd like
Automate the fallback to the last processed block height.

• The indexer should maintain a value somewhere indicating the last block it indexed.
• Update the config to allow one to choose starting from START_BLOCK_HEIGHT or from the interruption block

Additional context
Reference tutorial https://docs.near.org/build/data-infrastructure/lake-framework/running-near-lake/lake-start-options