synchronizer: double-check old L1 block to detect reorgs

[joanestebanr]

Rationale

Synchronizer is going to check in background the L1block after the block has been consolidated by ethereum.

You can choose what do you consider 'consolidated' (the safety level):

• type of block: finalized, safe or latest
• add or substract a offset

Implementation

There are 2 goruintes that are checking in background blocks:

• One check from 'consolidated' point and if it's ok it mark as checked the block and is no longer verified
• The other check from 'consolidated' point to newest blocks, it just verify without marking and do it continously until the block is marked as checked.

So you can configure next scenario:

• From 0 to safe: validate and mark as checked if match L1 RPC
• From safe to latest -5 : check in memory

Configuration

	[Synchronizer.L1BlockCheck]
		Enable = true
		L1SafeBlockPoint = "finalized"
		L1SafeBlockOffset = 0
		ForceCheckBeforeStart = true
		PreCheckEnable = true
		L1PreSafeBlockPoint = "latest"
		L1PreSafeBlockOffset = -32

• Enable : it enable of disable full feature
• ForceCheckBeforeStart: Before start anything it check the pending block up to 'consolidated' point
• L1SafeBlockPoint + L1SafeBlockOffset : define what we consider as 'consolidated' point
• PreCheckEnable: enables second gorutine that check newest blocks continuously
• L1PreSafeBlockPoint + L1PreSafeBlockOffset : define the maximum block to check for the second gorutine