Default max_concurrent_processing to half the CPUs pixa sees (closes #242)

The default was one image per CPU Go uses, and a container with no CPU
limit sees every CPU of the host. It is now half of runtime.GOMAXPROCS(0),
rounded down and at least 1, from imageprocessor.DefaultMaxConcurrentProcessing,
which both the config and imageprocessor.New use. The two existing tests of
the default expect the new one, as the owner allowed on the issue.

"Running under upaas" gives the worst-case memory, with a per-image peak of
about 1.6 GiB measured on pixad for an 8192x8192 source in each output
format, and tells the operator to set PIXA_MAX_CONCURRENT_PROCESSING and
PIXA_UPSTREAM_CONNECTIONS to fit the container's memory.

Model: opus-5-5

internal/imageprocessor/max_concurrent_processing_internal_test.go

@@ -115,14 +115,14 @@ func waitForEntries(t *testing.T, entered <-chan struct{}, count int) {
 	}
 }
 
-func TestNewDefaultsMaxConcurrentProcessingToCPUs(t *testing.T) {
+func TestNewDefaultsMaxConcurrentProcessingToHalfTheCPUs(t *testing.T) {
 	t.Parallel()
 
 	for _, limit := range []int{0, -1} {
 		proc := New(Params{MaxConcurrentProcessing: limit})
-		if got := cap(proc.processingSemaphore); got != runtime.GOMAXPROCS(0) {
-			t.Errorf("MaxConcurrentProcessing %d: %d slots, want %d, one per CPU",
-				limit, got, runtime.GOMAXPROCS(0))
+		if got := cap(proc.processingSemaphore); got != max(runtime.GOMAXPROCS(0)/2, 1) {
+			t.Errorf("MaxConcurrentProcessing %d: %d slots, want %d, half the CPUs",
+				limit, got, max(runtime.GOMAXPROCS(0)/2, 1))
 		}
 	}