Just-in-time liquidity for SPV vaults

[adambor]

Currently with the implementation of connector based spv vault swap protocol, we require that the bitcoin transaction funding the spv vault is properly finalized on the bitcoin blockchain before opening up the vault on the smart chain side and depositing funds to it, the client that is swapping also checks whether the vault UTXO returned by the LP is confirmed before accepting the quote.

However this can be changed without any impact on security of both LP and the user, in a following way:

• Since LP can be sure that his vault opening transaction will confirm (since the only one who can double-spend it is the LP itself - considering they use already confirmed inputs), we can let the LP open up the vault and deposit funds into it BEFORE the transaction on the bitcoin side confirms
• Users can accept vault UTXOs that are yet unconfirmed, considering that they are equal to the latest vault UTXO as specified in the current vault state on the smart chain

Extra care needs to be taken about the max mempool descendants/ancestors and their size, especially if the funds to fund the UTXO vaults were previously sent to some user (as part of smartchain -> BTC swap) - i.e. giving that user a possibility to exceed the mempool limits by crafting a big transaction (in that case the LP and the User would have to wait till the utxo vault creation TX confirms before being able to do any actions on it)