near: traverse block range via hashes

[heyitaki]

As pointed out by @jumpsiegel and @tbjump, it's technically possible to retrieve the incorrect block from an RPC node when fetching by height since blocks are finalized once 66% of the network accepts them. Therefore, if we somehow hit a node with a different history, the same block height may point to a different (unfinalized!) block. In addition, NEAR sometimes has gaps of permanently unfinalized blocks, disallowing a naive iteration over blocks heights.

Since there is no way to check if a specific block is finalized or not by block height, a possible solution may be to grab the hash of the latest finalized block and walk backwards to fromBlock using the prev_hash field.

[heyitaki]

Resolved by #34