⚗️ Create basic expiry model of RFC003

[LLFourn]

Calculate the appropriate alpha_expiry and beta_expiry for each party given the following:

• A function which when given the time since broadcasting a transaction and the transaction fee will return the probability that the transaction has been included in a block.
• A function which when given the time a transaction has been included in a block will return the probability of it being permanently in the blockchain.
• The party's required confidence that they won't lose an asset.

DOD:

• Make a meeting to demonstrate the model
• Give realistic alpha_expiry, beta_expiry values for BTC-ETH and ETH-BTC.
• Include results in relevant RFCs.
• Create issue to default to such realistic values in COMIT-i

Child of comit-network/comit-rs#670