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

README.md

@@ -83,10 +83,12 @@ which answers 200 whenever pixa is running, in maintenance mode too (see
 `maintenance_mode`).
 
 On SIGTERM or SIGINT pixa stops accepting connections, gives the requests in
-progress and the images being processed 5 seconds to finish, and exits: with 0,
-or with 1 when images were still being processed after those 5 seconds or
-another part of pixa failed to stop. A request not finished by then is cut off.
-`docker stop` waits 10 seconds before it kills the container.
+progress, the images being processed and the counts of cache hits, misses,
+fetches and conversions being written to the database 5 seconds to finish, and
+exits: with 0, or with 1 when images were still being processed or counts still
+being written after those 5 seconds, or another part of pixa failed to stop. A
+request not finished by then is cut off. `docker stop` waits 10 seconds before
+it kills the container.
 
 Outside Docker, pixa needs libvips (the image has 8.16) and libheif to run, as
 it uses libvips through CGO. pixad does not start unless libvips has its JPEG XL