source-backend: don’t use closed network connection
dcs-web: log connection close reasons to source backends
dcs-web: clean up -query_results_path to have 20% available disk space

…instead of hard-coding 10GiB, which breaks if you place
-query_results_path e.g. on a smaller /tmp file system (for testing).
source-backend: gracefully handle disconnects (query cancellation)

This should fix the unexpected source-backend task restarts.