This is enlarged because sometimes leader is repeatedly ticked
to trigger a desired heartbeat to new node, which may lead to
leader steping down if it's too small.

This does not prolong UT execution time because we do not rely
on ticks to trigger election.

Change-Id: I4957808fa73275e1932345f1f7e9c7930d6f696d
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>