Print interim statistics to STDERR on demand

[salsferrazza]

One could run the transcoder with debug options, but this slows down processing since the output is a function of a file. In some instances, one might desire seeing where a txcode process is with it's current batch non-destructively. One way to do this might be trapping kill -s INFO signal in main thread and then dump a snapshot of the progress up to that point. The dd command has a similar feature:

Sending an `INFO' signal to a running `dd' process makes it print
I/O statistics to standard error and then resume copying.  In the
example below, `dd' is run in the background to copy 10 million blocks.
The `kill' command makes it output intermediate I/O statistics, and
when `dd' completes, it outputs the final statistics.

     $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
     $ kill -s INFO $pid; wait $pid
     3385223+0 records in
     3385223+0 records out
     1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
     10000000+0 records in
     10000000+0 records out
     5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s