Flip locators endianness

[sr-gi]

Currently we are constructing locators using the big-endian representation of the dispute txid, given data is always queried to bitcoind and that's the format used to display data.

However, transaction ids are internally represented as little-endian, so this may be an issue when trying to integrate with other software that, internally, represents sha256d hashes in the same way as bitcoind does.

Therefore, locators may be better computed using the internal endianness representation (i.e. little-endian).

PS: double-checked with @roasbeef and lnd does compute hints using little-endian txids.