fix: closes issue #1071 - fetch a poi from the latestBlock if it's behind chainhead

[dwerner]

From the arbitration charter:

If a subgraph encounters an error that prevents it from being synced up to a the first block of the epoch, then an Indexer should submit the last valid PoI for the block before the error occured. Indexers may continue to collect indexing rewards on a broken subgraph until the market indicates it is no longer useful to index that subgraph and removes all signal from the subgraph.

This PR implements checking if the PoI exists at the last block that the subgraph is synced to when trying to unallocate, if we can't find a PoI in the 'first-block-of-current-epoch' and the subgraph synced lastBlock as reported by graph node is behind chainhead.