Replace workers std{out,err} by pipes and log anything written to them

This will help us identify problems in libnotmuch and reduces visual
artifacts (unfortunately libnotmuch writes messages to stderr that
used to clobber the curses interface).

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Add some comments to the db manager async code

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Merge branch '0.3.3-workers-love-555'