In etcdraft UT, we often need to deterministically elect a leader.
This was done by ticking ONLY one node in the network, so it is
the only node that start campaign.

HOWEVER, there are several problems with this approach:
1. it's slow. We need real time interval between ticks due to the
   way fake clock is implemented: it drops tick on the floor in
   case of slow consumer.
2. there is random factor in election timeout of etcd/raft. It is
   calculated as follow:
```
randomElectionTimeout = electionTimeout + rand.Intn(electionTimeout)
```
   in another word, if we send electionTimeout ticks, it's not
   guaranteed to trigger a leader election
3. if CheckQuorum is enabled, a lease is imposed on follower nodes
   which gets expired if
      electionTimeout <= elapsedTicks < randomElectionTimeout
   (if it's greater than randomElectionTimeout, it's reset to 0 and
   node starts campaign)

In this CR, we send an artificial MsgTimeoutNow to the node to be
elected. This message reliably triggers campaign and skip the lease
check.

This CR also fixes several potential data race and flakes in tests.

Change-Id: I3c8e0bcadbb8cfa1ae3393de2ea711fdd0d8b7aa
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>