shared event executor group between server and clients

- improved documentation
- closed memcache client's thread pools

rbcs-api/src/main/java/module-info.java

@@ -4,6 +4,7 @@ module net.woggioni.rbcs.api {
     requires io.netty.buffer;
     requires io.netty.handler;
     requires io.netty.transport;
+    requires io.netty.common;
     exports net.woggioni.rbcs.api;
     exports net.woggioni.rbcs.api.exception;
     exports net.woggioni.rbcs.api.message;