Add API waitNodes to wait for cluster nodes to join

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Add API unsafeUpdate to allow update commited ballot

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Probe for lease in random interval

In previous implementation, pdLease probe for lease according to its
expire time, but it's possible that the owner release the lease before
expire time so in this patch we probe for lease in a random interval in
case the owner releases before expire.

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Update basic distributed test

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>