Client / server code for handling histograms. The server:

- Deals with subtracting consecutive histograms so that client
  doesn't have to recreate linked list.
- Uses existing IOLOG command code to create packets, checking
  for IO_LOG_TYPE_HIST when necessary.

And the client:

- Reconstructs the pointers to the histogram bins from the packet
  format, namely (sample_0, hist_0, sample_1, hist_1, ...) in lieu
  of the current (sample_0, sample_1, ...) format used for the (void *)
  cur_log->samples.
- Flushes histograms to file with updated pointer calculation to get
  ith sample, disabling subtraction in hist_sum() using a null
  pointer.

This does not cover plain-text transmission mode (i.e. when zlib is
not present during fio compilation).

Signed-off-by: Karl Cronburg <kcronbur@redhat.com>