This gets truncated for on-chain transactions (thus, rounding may
contribute to fees).
This also means we currently have an upper bound of 0.04 BTC per HTLC;
this can be increased later if required.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Most complex change was gather_updates(), which handles all the "what
is the current state of the channel" logic for our dumb test utils.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
