Commitment of block is doen aync win blockwritter, therefore
`support.Height` may not correctly reflect the actual number
of latest block. Decision made based on `support.Height` while
chain is running may lead to replaying of same block.

This CR fixes this by keeping a reference to the last block,
which is initialized by reading the tail of ledger at start.

Change-Id: I6c5e5fed4c1464c459603f4484a44b8b91b017b6
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>