No period data on testnet sentinels for older UTXOs

[troggy]

The new code to generate CAS proofs relies on period data availability. We've started collecting period data on a leap-node side just recently, so it is available for newer periods. However, sentinels were not resynced from the beginning, so there is no data collected for the older periods — hence the generic getProof helper will fail for older utxos.

Possible solutions:

1. Resync sentinels, so there is a data for all the periods.
2. Workaround this on a bridge UI side, by falling back to older proof generation code if there is no period data (or for heights < X)