Allow for out-of-order transaction destroys
ATS: Force notify request / response finished before destroy
modhtp: Handle NULL request / response headers
Engine various: Handle NULL request / response header
modhtp: Don't trigger _body_data_ex() with NULL if there has been
no acutal body data.
ATS plugin: Don't trigger body data for zero-length blocks.
ib_tx_destroy(): Fixed several bugs in the tx list handling introduced
in previous commit.
httpd: Add some asserts and extra logging into the Apache httpd server.