cooker.py: Improve server idle handling (from Poky)

cooker.py: Improve server idle handling (from Poky)

This patch:
  * Uses the xmlrpc timeout controls added in python 2.6 removing the need to
    hack the class instead. The hacking didn't work for 2.6.
  * Converts the parsing function to parse a file per call to the function
  * Allows checking for xmlrpc commands between parsing files allowing the user
    to interrupt parsing.
  * Starts to teach the command queue about command properties such as whether
    they require the cache.

Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>