Msgscan missed some logs using eth_getLogs

[wuminzhe]

The logs indexing task of msgscan missed some logs before adding an extra scan.

code: https://github.com/darwinia-network/pindex/blob/e5521205757278baebd720f972cfc9bffb32f461/lib/tasks/logs.rake#L17

rpc: https://github.com/darwinia-network/pindex/blob/e5521205757278baebd720f972cfc9bffb32f461/config/pindex.yml#L21

[wuminzhe]

To mitigate this problem, I added an extra scan a little behind the head. The head here means the latest block number - 6.
code: https://github.com/darwinia-network/pindex/blob/e5521205757278baebd720f972cfc9bffb32f461/lib/tasks/logs.rake#L26C1-L31C10

[hackfisher]

duplicate with #1473