Additional LWLOCK_STATS statistics

Hi,

I have been using the attached patch to look at how each LWLock relates
to each other in various types of runs.

The patch adds the following fields to a LWLOCK_STATS build:

  sh_acquire_max (int):

    The maximum shared locks in series for the lock

  ex_acquire_max (int):

    The maximum exclusive locks in series for the lock

  max_waiters (int):

    The maximum numbers of waiters

Also attached is a sample report using FlameGraphs from a pgbench run using

  -c 80 -j 80 -T 300

See README for additional details.

If there is an interest I'll add the patch to the next CommitFest.

Thanks for considering, and any feedback is most welcomed.

Best regards,
  Jesper