Fix for process check

* Properly compute context switches metrics
* Do not fail on getting file descriptors count

Fix #812 Fix #814
Do not fail the info page if it cannot get the ntp offset
Allow JMX checks to display the number of metrics they get on a per instance basis
Only start service check thread pool at first iteration of the check, not when it's initialized
Do not restart jmxfetch when running one of the "list" command
Allow to specify a list of checks to be used when using JMX list commands