Add option to start multiple workers on a single EC2 instance (feedback from
tutorial).
Move summary to common header.
Move resources monitoring to poll library.

Move mem, io, cpu-time, wd, proc/wd/fs monitoring to poll library.
Reports only the original limit exceed.

When a limit is exceeded, the line written to the summary is, for
example:

limits_exceeded: wall_time: 2, num_processes: 10

Before it was:
limits_exceeded: wall_time: 2.02 > 2, num_processes: 11 > 10

This makes the limits_exceeded global variable unnecessary.
Print summary with a single function call.
Adds total count of process to summary/series.

num_processes counts the number of different processes created,
concurrent or not.

max_concurrent_processes was renamed to max_processes.
Renames workdir_number_files_dirs to workdir_num_files.
Enable disk footprint for cctools hook by default.
Consistent treatment of log command line flags.
Cleans up parsing of limits.
Merge branch 'polling-library'