Server crashes with ConcurrentModificationException

[QUICTester]

Hi,

During our tests involving a Quiche4j implementation with the commit version ea5effc, we identified 1 fault:

The example server crashes with ConcurrentModificationException when the client tries to set up multiple connections with the server using a constant IP address and port number. The root cause of this crash is the server tries to remove a client from the HashMap while looping through the HashMap (line 323) in quiche4j/quiche4j-examples/src/main/java/io/quiche4j/examples/Http3Server.java:main().

The server should not execute clients.remove(connId) in the for loop.