The %>handshake logformat code (#243)

The %>handshake logformat code (#243)

Logging client "handshake" bytes is useful in at least two contexts:

* Runtime traffic bypass and bumping/splicing decisions. Identifying
  popular clients like Skype for Business (that uses a TLS handshake but
  then may not speak TLS) is critical for handling their traffic
  correctly. Squid does not have enough ACLs to interrogate most TLS
  handshake aspects. Adding more ACLs may still be a good idea, but
  initial sketches for SfB handshakes showed rather complex
  ACLs/configurations, _and_ no reasonable ACLs would be able to handle
  non-TLS handshakes. An external ACL receiving the handshake is in a
  much better position to analyze/fingerprint it according to custom
  admin needs.

* A logged handshake can be used to analyze new/unusual traffic or even
  trigger security-related alarms.

The current support is limited to cases where Squid was saving handshake
for other reasons. With enough demand, this initial support can be
extended to all protocols and port configurations.

This is a Measurement Factory project.