private static long getNextId() {
        // (1000 + 1) ~ (1000 + 8191)
        return (sequence.incrementAndGet() & ID_MASK) + ID_BASE;
    }
Gets current pid, max pid 32 bit systems 32768, for 64 bit 4194304
http://unix.stackexchange.com/questions/16883/what-is-the-maximum-value-of-the-pid-of-a-process
http://stackoverflow.com/questions/35842/how-can-a-java-program-get-its-own-process-id
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/util/UniqueIdUtil.java