feat: simplify single node cl implementation

[shaspitz]

The current implementation of our consensus layer is exhibiting odd behavior where recently built block timestamps will jump up to 20 minutes ahead of system time. This PR streamlines implementation, removes unnecessary variables, and makes the logic easier to comprehend/debug. After testing, these changes have fixed the issue.

• bb.lastBlockTime variable was being set independently from bb.executionHead.BlockTime. We now use existing bb.executionHead.BlockTime in-place
• head variable local to GetPayload was being set independently from bb.executionHead. We now use existing bb.executionHead in-place
• loadExecutionHead was removed. We now just set bb.executionHead once upon service restart in setExecutionHeadFromRPC and subsequently update bb.executionHead at the end of every FinalizeBlock
• Block timestamp computation was simplified
• bb.buildDelayMs removed, bb.buildDelay remains