Bound concurrent image processing and upstream fetches (closes #64)

max_concurrent_processing (default: the number of CPUs Go uses) bounds
the images processed at once, and upstream_connections (default 64) the
fetches from all upstream hosts together, beside the per-host limit. A
request that finds either full waits up to 10 seconds, then gets 503
"server busy, try again later". The processor holds its slot from before
it reads the input until it returns, and takes a free slot even after the
request context has ended; a fetch holds its connection until the
response body is closed, after its image is processed. libvips now starts
with one worker thread per image and no operation cache. Both settings
have PIXA_ variables and are in README.md and config.example.yml.

Model: opus-5-5

README.md

@@ -235,6 +235,8 @@ variables set by the file's `env:` section are checked the same way.
 | `PIXA_TRUSTED_PROXIES`               | `trusted_proxies`               | CIDR ranges of proxies whose `X-Forwarded-For` is believed; default RFC 1918 |
 | `PIXA_ALLOW_HTTP`                    | `allow_http`                    | Allow plain-HTTP upstreams, for testing only; default `false`                |
 | `PIXA_UPSTREAM_CONNECTIONS_PER_HOST` | `upstream_connections_per_host` | Concurrent connections per upstream host; default `20`                       |
+| `PIXA_UPSTREAM_CONNECTIONS`          | `upstream_connections`          | Concurrent connections to all upstream hosts together; default `64`          |
+| `PIXA_MAX_CONCURRENT_PROCESSING`     | `max_concurrent_processing`     | Images processed at once; default the number of CPUs                         |
 | `PIXA_METRICS_USERNAME`              | `metrics.username`              | Username for `/metrics`, which is served only when both are set              |
 | `PIXA_METRICS_PASSWORD`              | `metrics.password`              | Password for `/metrics`; set together with the username                      |
 | `PIXA_SENTRY_DSN`                    | `sentry_dsn`                    | Sentry DSN for error reporting; empty disables it                            |
@@ -276,6 +278,16 @@ Key settings in more detail:
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the
   disk cache entirely; omitted defaults to 75% of the free space on
   the filesystem containing `<state_dir>/cache/` (minimum 500 MiB)
+- `upstream_connections` — the most connections to upstream hosts at once, all
+  hosts together, on top of `upstream_connections_per_host`; default `64`. A
+  fetch holds its connection until its image has been processed. A fetch that
+  finds all of them in use waits up to 10 seconds for one to free up; if none
+  does, the request is answered 503 with the error
+  `server busy, try again later`
+- `max_concurrent_processing` — the most images decoded and encoded at once;
+  default the number of CPUs pixa can use (`GOMAXPROCS`), which follows a
+  container's CPU limit. A request that finds all of them in use waits up to 10
+  seconds for one to free up; if none does, it is answered 503 the same way
 
 See `config.example.yml` for all options with defaults.