In-protocol liquidity probing and channel jamming mitigation

Though free messaging is already possible via htlcs, a probe message would lower the cost to do so because the sender wouldn't need to lock up liquidity. This could increase the spam potential. It is suggested that the probe message be designed such that it is useless for anything other than probing. However, this may be difficult as the obfuscated 1300 bytes block with the remaining part of the route cannot differentiate between meaningful data and other data. To mitigate this issue, the onion max size for the probe can reduce, making it less usable for remote messaging.