Issue description

The parameter-less ByteBody.split() method uses SplitBackpressureMode.SLOWEST. This mode can lead to deadlocks that are difficult to spot. We should force users to specify the backpressure mode instead.

• The parameter-less ByteBody.split method should be deprecated in 4.10 and removed in 5.0
• The same method should remain in AvailableByteBody where deadlocks are not an issue