This CR removes the global leader var in etcdraft chain because
it is racy in following case: several requests are to be enqued
into submitC while leader loses its leadership.

This also removes the lock on rpc.SendSubmit because it's guarded
by the channel.

Change-Id: If5e785e05dcf9bfc60e403f2d5813baf769ee103
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>