ALG message buffer optimization

A message buffer is allocated only when the packet is ready to process. The buffer is freed after the packet completes ALG handling, including modifying the payload, performing NAT, opening a pinhole for a new connection between a client and a server, and transferring data between a client and a server located on opposite sides of a Juniper Networks device. 

This feature has the following enhancements: 

•  Unnecessary objcache buffering is avoided, resulting in low memory utilization. 
•  jbuf manipulation is used to simplify the message buffer logic. 
•  Full-fledged message buffer support for the ALG line breaker is more flexible. 
•  ALG Manager and ALG plug-in logic clarity are optimized.