Store blockchain seqs and unspents in boltdb

[iketheadore]

Description

For now, we have block tree struct to store all blocks in db, each depth of the tree may have multiple blocks, we use consensus algorithm to determine the path and form the blockchain.

Each time we start the node, will go through the block tree and get the blockchain head and unspent pool, which is a waste of time. So we now decide to store unspents and block head in boltdb, which will speed up our wallet opening speed.

[iketheadore]

Fixed in #363