…uring reassignment (apache#7795)

KIP-320 improved fetch semantics by adding leader epoch validation. This relies on
reliable propagation of leader epoch information from the controller. Unfortunately, we
have encountered a bug during partition reassignment in which the leader epoch in the
controller context does not get properly updated. This causes UpdateMetadata requests
to be sent with stale epoch information which results in the metadata caches on the
brokers falling out of sync.

This bug has existed for a long time, but it is only a problem due to the new epoch
validation done by the client. Because the client includes the stale leader epoch in its
requests, the leader rejects them, yet the stale metadata cache on the brokers prevents
the consumer from getting the latest epoch. Hence the consumer cannot make progress
while a reassignment is ongoing.

Although it is straightforward to fix this problem in the controller for the new releases
(which this patch does), it is not so easy to fix older brokers which means new clients
could still encounter brokers with this bug. To address this problem, this patch also
modifies the client to treat the leader epoch returned from the Metadata response as
"unreliable" if it comes from an older version of the protocol. The client in this case will
discard the returned epoch and it won't be included in any requests.

Also, note that the correct epoch is still forwarded to replicas correctly in the
LeaderAndIsr request, so this bug does not affect replication.

Reviewers: Jun Rao <junrao@gmail.com>, Stanislav Kozlovski <stanislav_kozlovski@outlook.com>, Ismael Juma <ismael@juma.me.uk>