Improve loading time with many blocks

[ineiti]

If there are many blocks (3000 for 10k votes), startup of a node takes about 1 minute. This is due to the method here:

https://github.com/c4dt/dela/blob/main/core/ordering/cosipbft/blockstore/disk.go#L70

It crawls through all blocks, deserializes them, and creates a map of block-hashes to block-indexes. Some comments:

• this index seems to be never used...
• one could store this map in another part of the db, so startup would be much faster
• what is used is the cachedData.last and cachedData.length, so only store those in the db