MINOR: htx: Add a field to save the headers data size

MINOR: htx: Add a field to save the headers data size

The size of headers present in an HTX message are now counted. A dedicated
field was added in the HTX structure to do so. This patch is quite simple
but it will be mandatory to be able to perform some tests on headers. One of
them is to be sure headers never exceed the regular buffer size, even when a
large buffer is used.