Disable the use of the Invocation Counter Plugin in our test cases to make
assertions about the number of times that various kinds of plugins were
invoked.  A number of test cases making assertions about the invocation counts
were failing pretty frequently, and after analyzing the problem it is not
something that can be easily addressed.

The underlying causes include:
- Processing on unbind operations continuing in a separate thread from a
  previous test case.  Whenever the client sends an unbind request to the 
  server, it immediately disconnects and moves on to the next test case.
  However, the server may continue to read and process that unbind operation
  (including invoking plugins on it) in a separate thread.

- For modify DN operations, the referential integrity plugin was sometimes
  responsible for invoking operations in the background that could increase the
  invocation count.