Running Omni node in prune mode

[bvbfan]

Currently Omni node requires full bitcoin data including tx index. According to https://ycharts.com/indicators/bitcoin_blockchain_size blockchain size is about 470GiB, tx index is more about 30GiB. It can be seen a tendance, last 3 months, to increase a bit more (probably Ordinals) so it will be helpful Omni layer to offer a lightweight node to individuals or testing purposes.

In order to address the issue, Omni node could allow to be ran in prune mode, Omni transaction database could be extended to record all transactions related to the protocol. Possible solution is to create db index {block height, block index, cheap hash} to handle reorgs/reverts and {tx hash, tx data} to replace tx index requirement.