public long getLastHeartbeat() {
        return lastHeartbeat;
    }
Returns the last time a heartbeat was received. Hearbeats are represented as whitespaces
or \n characters received when an XmlPullParser.END_TAG was parsed. Note that we
can falsely detect heartbeats when parsing XHTML content but that is fine.

@return the time in milliseconds when a heartbeat was received.
xmppserver/src/main/java/org/jivesoftware/openfire/net/MXParser.java