refactor(general): minor miscellaneous cleanups (#4074)

Cleanups:

- use non-format variants of Log/Print with no additional args;
- fold in Fprintf call with no args into the following one;
- add missing arg placeholder in format strings;
- use require.Positive instead of Greater(..., 0);
- rename function to fillWithZeros to avoid collision with builtin clear;
- define type for context key to avoid collisions.

snapshot/snapshotfs/upload_test.go

@@ -740,7 +740,7 @@ func TestParallelUploadUploadsBlobsInParallel(t *testing.T) {
 
 	require.NoError(t, th.repo.Flush(ctx))
 
-	require.Greater(t, maxParallelCalls.Load(), int32(0))
+	require.Positive(t, maxParallelCalls.Load())
 }
 
 func randomBytes(n int64) []byte {