utils: Update find to be threaded.

This is needed to speedup searches when using network mounted directories where
we are heavily IO bound.
util: Add find_mtimes helper
local: Update how scanner work is done to improve remote FS support.

This changes the scanner to prefetch all the files+mtimes using our new
threaded finder. Hopefully making scanning against remote filesystems
somewhat less painful.
utils: Remove old find_files
Merge pull request #707 into develop

Improve scan performance for remote filesystems
utils: Tweak docstrings from PR#707
docs: Update changelog