The cost cleanup migration pages through LND payments only to build
a payment-hash to fee map. It does not inspect HTLC attempts,
routes, or per-hop data; pagination still uses the top-level index
offsets returned by ListPayments.
Setting OmitHops is safe for this migration because LND only strips
hop-level route data from HTLC attempts, while preserving the top-level
payment fields the migration reads: hash, fee, and response offsets.
This reduces response size and query cost for nodes with many or large
MPP payments without changing the calculated swap costs.
The migration test records the mocked ListPayments requests and asserts
that OmitHops is set.
Previously we defaulted to fetching 1000 payments at a time when running
the cost migration however due to slowness on LND side this may prevent
the migration from completing due to RPC timeout kicking in. This commit
adds a new config value to give more control to the user by setting the
batch size as needed.
Previously we may have stored negative costs for some loop out swaps
which this commit attempts to correct by fetching all completed swap,
calculating the correct costs and overriding them in the database.
