title: what is cxid used for in Zookeeper
body: <p>in the zookeeper logs and also in source code i see there is a field named <code>cxid</code> in Request/Txn/Header.</p>
  
  <pre><code>2018-06-24 02:28:55,003 [myid:1] - DEBUG [CommitProcessor:1:FinalRequestProcessor@88] - Processing request:: sessionid:0x1642e63d6060000 type:ping cxid:0xfffffffffffffffe zxid:0xfffffffffffffffe txntype:unknown reqpath:n/a
  </code></pre>
  
  <p>i am not able to understand what it is used for.</p>