Improve service start-up performance / experiance

[haku]

currently on start up items appear to not exist until filesystem has been re-scanned.
this can be delayed if new files need to be hashed are ahead of files that are already know about.

ideas:

• move file-hashing to a lower priority queue (easier after 4680c08)
• somehow rebuild ContentTree based on DB before scanning file system?

[haku]

d7241e4 did that first part, which should make things smoother.