Add support for parallel --regen, using the existing --jobs and --load-average

Add support for parallel --regen, using the existing --jobs and --load-average
options:

 * Split out a PollLoop base class from Scheduler and derive a MetadataRegen
   class to handle parallel scheduling for --regen.

 * Add pordbapi._metadata_process() and _metadata_callback() methods to
   implement asynchronous metadata generation. One method returns an
   EbuildMetadataPhase instance to encapsulate the async task. The other
   method is called to save the metadata when the process completes
   successfully. These methods share code with aux_get to avoid duplicate
   code.

svn path=/main/trunk/; revision=11012