mv task start datetime out of sessions; add in task list view

We want the task start attrib to be a datetime so templates can
display it, but the datetime causes task serialization to crash.
Adding the startd datetime in webui.tasks.session_tasks_list()
lets the datetime still get added to the session, which cannot
be serialized by Redis.
This change removes the datetime from sessions_tasks_list and
adds it in the tasks list view.
Merge branch 'master' into 099-inventory
fixed wrong name for command.annex_status
collection annex-whereis and annex-map funcs; add to status view

This change adds annex-whereis and annex-map functions to
webui.models.Collection, and adds them to the collection status
page.  These are expensive to calculate so they are cached.
increased timeouts for cached annex metadata
collection cache_refresh function and celery task; add to views

Delete cached collection metadata and do the time-consuming
regeneration in the background.