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.
