perf: enable uuid randomness pool and minor cleanups (#1652)

cmd/anubis: call uuid.EnableRandPool() at the top of main. The pool
batches crypto/rand reads internally, dramatically reducing per-call
syscall overhead for UUID generation. UUIDs are produced on every
issued challenge (NewV7, 3.7 times faster, down to zero allocation) and on
every challenge page render (NewString, 1.6 times faster, 1 fewer allocation).
The pool is non-cryptographic-key material, PoW challenge bytes and signing
keys still go directly to crypto/rand.

lib/anubis.go: three trivial optimizations in issueChallenge and
maybeReverseProxy, reducing the amount of allocations by 2%, which isn't much
but since the changes are trivial:

  - fmt.Sprintf("%x", randomData) -> hex.EncodeToString(randomData)
  - cache uuid.UUID.String() once instead of calling it three times
  - fmt.Sprintf("ogtags:allow:%s%s", ...) -> string concat

Signed-off-by: jvoisin <julien.voisin@dustri.org>
Signed-off-by: Xe Iaso <xe.iaso@techaro.lol>
Co-authored-by: Xe Iaso <xe.iaso@techaro.lol>

cmd/anubis/main.go

@@ -36,6 +36,7 @@ import (
 	"github.com/TecharoHQ/anubis/lib/thoth"
 	"github.com/TecharoHQ/anubis/web"
 	"github.com/facebookgo/flagenv"
+	"github.com/google/uuid"
 	_ "github.com/joho/godotenv/autoload"
 	healthv1 "google.golang.org/grpc/health/grpc_health_v1"
 )
@@ -193,6 +194,9 @@ func main() {
 	flagenv.Parse()
 	flag.Parse()
 
+	// Must be set before any concurrent UUID call.
+	uuid.EnableRandPool()
+
 	if *versionFlag {
 		fmt.Println("Anubis", anubis.Version)
 		return