Rewrite Flock more aggressively #3375

Revert the following commits, then add minimal patch:

Revert "edit documentation of new knob FLOCK_INCREMENT" #3375

This reverts commit 6e1444fc2a7ee3a2a666274b229475f1cc4bf80a.

Conflicts:
	doc/version-history/7-9.history.tex

This reverts commit 0d408d879360da70a756976b6b4c88450dfd330f.

Conflicts:
	doc/version-history/7-9.history.tex
	src/condor_schedd.V6/schedd.cpp
Use random flocking increment #3484