Stop waiting for count writes after a request's deadline (closes #224)

TestService_Get_ReturnsByItsDeadline failed on a busy host because its
request, past its deadline, still waited for its miss count to be
written, a database write with no deadline; in pixad that write waits
for the one database connection every request shares. The hit, miss,
upstream fetch and transform counts are now each written in a goroutine
of their own, which the request waits for only until its deadline; a
count not written by then is written after it returns. Shutdown waits
for those writes within ShutdownTimeout and reports any left unfinished.
The test phase also runs go test with -parallel 4: on a busy host, as
many tests at once as there are CPUs wait so long to be scheduled that
a timed request can fail.

Model: opus-5-5

internal/handlers/handlers.go

@@ -93,6 +93,13 @@ func (s *Handlers) WaitForProcessing(ctx context.Context) int {
 	return s.imgSvc.WaitForProcessing(ctx)
 }
 
+// WaitForCountWrites waits until every count a request has started writing
+// to the database is written, or until ctx ends, and reports whether they
+// all were.
+func (s *Handlers) WaitForCountWrites(ctx context.Context) bool {
+	return s.imgSvc.WaitForCountWrites(ctx)
+}
+
 // newCacheConfig builds the image cache's configuration from cfg.
 // cache_max_bytes: 0 disables the disk cache entirely; any other value
 // is the eviction limit in bytes; when it is omitted, the cache works