Previously, we protected from multiple ProposerElected messages from the same
walproposer with the following condition:
msg.term == self.get_last_log_term() && self.flush_lsn() >
msg.start_streaming_at
It is not exhaustive, i.e. we could still proceed to truncating WAL even though
safekeeper inserted something since the divergence point has been
calculated. While it was most likely safe because walproposer can't use
safekeeper position to commit WAL until last_log_term reaches the current
walproposer term, let's be more careful and properly calculate the divergence
point like walproposer does.
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