Configurable starting queue index

Each producer and consumer uses an integer index to identify the next
slot in the ring buffer to write into or read from.  Before, we used a
very large initial index, to make sure that the index overflows (and
wraps around to 0) at some point during the test.  This makes the
debugging messages hard to read during normal operation, however, since
the numbers all look pretty similar.  Now, the default is to start each
queue index at 0 (to make them easy to read), and provide a function
(`vrt_testing_mode`) to use the high starting value in test cases.
Debug messages using clogger

All of the existing debug messages (which are normally ifdef-ed out)
have been replaced with clogger debug messages.  Most of them are
trace-level messages, but we have debug-level messages that can be used
to see when producers and consumers have to yield.  This should
(hopefully) be useful in detecting deadlocks.
Always collect queue statistics

They involve extra increments, which are not at all expensive.  And the
data is almost certainly useful.
Merge branch 'feature/debugging' into develop

* feature/debugging:
  Always collect queue statistics
  Debug messages using clogger
  Configurable starting queue index