Protocol desynchronization after Connect for (some) old clients

Some old clients experience a protocol synchronization after receiving a ConnectResponse from the server.

This started happening with ZOOKEEPER-4492, "Merge readOnly field into ConnectRequest and Response," which writes overlong responses to clients which do not know about the readOnly flag.

(One example of such a client is ZooKeeper's own C client library prior to version 3.5!)