paraphotos: simplify workers waiting for sync
paraphotos: simplify workers waiting for sync

use semaphore for letting go of downloader tasks when having notified
the user, instead of keeping track of notification counter index in each
worker.

the sync is needed because otherwise less than N downloaders might do
more downloads than others and disguise an "all loaders at same point"
situation.

sooda/thesis,sooda/thesis,sooda/thesis,sooda/thesis