Improve scalability of counters a lot

Counters now scale orders of magnitudes better as the number of
writers to the same counter increases. Here's the results on our
current benchmark on a 6 core machine:

        cores
            1       6
before  1.87s  81.09s
after   0.11s   0.32s

Note how the very heavy contention gives a slowdown rather than a
speedup as the number of cores increases, but the new implementation
slows down much less than the old one and is also generally faster.