public static UniqueRequestId next(String serverId) {
        return new UniqueRequestId(serverId, System.currentTimeMillis(), sequenceCounter.getAndIncrement());
    }
Creates a session id which is unique across the cluster this runtime is a member of each time this is called.
Calling this causes synchronization.
container-search/src/main/java/com/yahoo/search/query/UniqueRequestId.java