Currently the teleportr database supports a LatestDeposit method,
which returns the highest block number observed, if any. The intent was
to use this as the starting point for syncing, however, this isn't super
useful as there may be long periods of inactivity that we have already
scanned.

Instead, we now store the last processed block in a separate table, and
pass the end of the ingestion block range as argument to UpsertDeposits.
The list of deposits and last processed block are written atomically to
avoid consistency issues. The value can be retrieved using the
LastProcessedBlock getter.