feat: Adjust API of submitchainlock

[PastaPastaPasta]

Talked to Sam; we want change the API in this way
First (if height <= bestchainlockheight return bestchainlockheight)
else submitchainlock then return bestchainlockheight

This provides two benefits; 1st is a short circuiting if we know the proposed chainlock could not overwrite the current one (due to lower height)
Second; returns the bestchainlockheight to platform

Originally posted by @PastaPastaPasta in #5765 (comment)