Generalize OTEL API and add memcache tracing support

- Rename RedisSpan -> SpanHandle for generic span handling
- Generalize TelemetryController methods: startSpan/endSpan with dbSystem param
- Rename RedisOtelSpan -> OtelSpanHandle in rbcs-server-otel
- Update Redis cache handler to use new generic API
- Add OpenTelemetry tracing for memcache GET and SET commands
- Add channel property to MemcacheRequestController for server address attribution
- Add uses TelemetryController directive in memcache module-info

Memcache spans follow the same pattern as Redis:
db.system=memcache, db.operation=GET|SET, server.address, server.port

rbcs-server-memcache/src/main/java/module-info.java

@@ -1,4 +1,5 @@
 import net.woggioni.rbcs.api.CacheProvider;
+import net.woggioni.rbcs.api.TelemetryController;
 
 module net.woggioni.rbcs.server.memcache {
     requires net.woggioni.rbcs.common;
@@ -16,5 +17,7 @@ module net.woggioni.rbcs.server.memcache {
 
     provides CacheProvider with net.woggioni.rbcs.server.memcache.MemcacheCacheProvider;
 
+    uses TelemetryController;
+
     opens net.woggioni.rbcs.server.memcache.schema;
 }