MEDIUM: htx: Harden HTX API to avoid invalid uses when EOM was reached

MEDIUM: htx: Harden HTX API to avoid invalid uses when EOM was reached

With recent changes, the info about the end of the message is now carried by
a block only. In addition, it is really important to never mix HTTP blocks
with tunneled data because we must be able at any time to test the tail
block to know if the EOM flag is set or not. To avoid any invalid uses, the
API was reinforced. BUG_ONs were added to trigger errors when such uses are
detected.

Note it should not be a huge change because mixing http data and tunneled
data was already not supported.