Gather information about computers while they are working on tasks

[kaazoo]

Imported from Redmine:
Information about computers is needed (current status for example) when they have running tasks. Another possiblity would be to cache that information and only update it from time to time.

Computer information is now cached in MongoDB for a configurable time. So the client program doesn't have to wait for the information task to complete while a computer is running real rendering tasks.

Hopefully IPython will support high priority tasks in the future, which would make this possible without caching.