Decrease the number of resource updates.

This commit forces resource updates on task reception, start of task
execution, and final task report. This allows to send 'unforced'
resource updates less frequently (here every two minutes, rather than
every five seconds). The idea is not to send resource updates if nothing
interesting has happened at the worker.
Merge pull request #443 from btovar/reduce_resource_updates

Send resource updates after events that might change the resource usage. And limit the time interval for automatic resource updates to 2 minutes.