[DMNs] Chainlock optimizations

[panleone]

This PR refactor and fixes bugs of the ChainLock class:

Refactor:

• Remove unused variable from UpdatedBlockTip
• Decouple init/destroy and start/stop steps as we do for all llmq classes
• Use LookUpBlockIndex in place of mapblockindex
• Decouple the chainlock signing part from UpdateBlockTip to a new function TrySignChainTip
• Make EnforceBestChainLock the only function where blocks can be invalidated
• Use a local scheduler instead of the global one

Bug fixes:

• Call EnforceBestChainLock only throught the scheduler to avoid recursions
• Don't ban peers if the received chainlock is not valid since it could simply means that the node is not fully synced yet
• Remove non used locks/ lock only when it's necessary
• Improve the functional test that was failing