pruner: remove retry handling + fix early give-up

Retry handling is broken since the gRPC changes (wrong error classification).
Will come back at some point, hopefully by merging the replication
driver retry infrastructure.

However, the simpler architecture allows an easy fix for the problem
that the pruner practically gave up on the first error it encountered.

fixes #123

daemon/pruner/pruner_queue.go

@@ -58,10 +58,7 @@ func (q *execQueue) Pop() *fs {
 func(q *execQueue) Put(fs *fs, err error, done bool) {
 	fs.mtx.Lock()
 	fs.execErrLast = err
-	if err != nil {
-		fs.execErrCount++
-	}
-	if done || (err != nil && !shouldRetry(fs.execErrLast)) {
+	if done || err != nil {
 		fs.mtx.Unlock()
 		q.mtx.Lock()
 		q.completed = append(q.completed, fs)
@@ -78,9 +75,6 @@ func(q *execQueue) Put(fs *fs, err error, done bool) {
 		defer q.pending[i].mtx.Unlock()
 		q.pending[j].mtx.Lock()
 		defer q.pending[j].mtx.Unlock()
-		if q.pending[i].execErrCount != q.pending[j].execErrCount {
-			return q.pending[i].execErrCount < q.pending[j].execErrCount
-		}
 		return strings.Compare(q.pending[i].path, q.pending[j].path) == -1
 	})
 	q.mtx.Unlock()