Raft config metadata checks, e.g. non-zero TickInterval, should also
be done against HeaderType_CONFIG transactions.

Also, since we check against nil consenter set, this also prevents
user from removing node from single node cluster, which results in
dead channel.

Change-Id: Idf0967ed19ffed964f16c83a9862c1c59e132686
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>