Take block reorgs in consideration in message relayer

[mertwole]

Message relayer works on chain head for now, so if it will submit some message to ethereum and block containing merkle root which references this message will get pruned, relayer will try to relay it again with the same(already pruned) block

To address this issue, latest_merkle_root shall be replaced with one that's included into latest not pruned block in the case described above