MINOR: htx: Add an HTX flag to know when a message is fragmented

MINOR: htx: Add an HTX flag to know when a message is fragmented

HTX_FL_FRAGMENTED flag is now set on an HTX message when it is
fragmented. It happens when an HTX block is removed in the middle of the
message and flagged as unused. HTX_FL_FRAGMENTED flag is removed when all
data are removed from the message or when the message is defragmented.

Note that some optimisations are still possible because the flag can be
avoided in other situations. For instance when the last header of a bodyless
message is removed.