Add keys for the Zxid from the stat command to check_zookeeper.py

Add keys for the zxid and its component pieces: epoch and transaction counter. These are not reported by the 'mntr' command so they must be obtained from 'stat'. The counter is useful for tracking transaction rates, and epoch is useful for tracking leader churn. zk_zxid - the 64bit zxid from ZK zk_zxid_counter - the lower 32 bits, AKA the counter zk_zxid_epoch - the upper 32 bits, AKA the epoch