Fix #22363: set max queue size to 100

See #39
See openanalytics/shinyproxy#65

Tested using siege performance test tool using 100 concurrent users
accessing a proxied resource.

Without this change 77% of the requests succeeds, with the change 100%
of the requests succeeds.

Author: LEDfan

src/main/java/eu/openanalytics/containerproxy/util/ProxyMappingManager.java

@@ -88,6 +88,7 @@ public void getConnection(ProxyTarget target, HttpServerExchange exchange, Proxy
 				super.getConnection(target, exchange, callback, timeout, timeUnit);
 			}
 		};
+		proxyClient.setMaxQueueSize(100);
 		proxyClient.addHost(target);
 
 		mappings.put(mapping, proxyId);