kv: lease extensions can be rejected due to raft follower status

Currently, the `propBuf` can reject extensions of an existing lease because the current leaseholder is not the raft leader. This was not the intention of the rejection logic in the `propBuf` and should be avoided.

This motivates a change like https://github.com/nvanbenschoten/cockroach/commit/b43772261af1bd515c66b84e4667313fc1faffd6.