ZOOKEEPER-2307: ZooKeeper not starting because acceptedEpoch is less than the currentEpoch

[symat]

Update acceptedEpoch and currentEpoch in file first then in memory.

Closes #1145 from arshadmohammad/ZOOKEEPER-2307-epochUpdate and squashes the following commits:

b05bc1f [Mohammad Arshad] review comment fix
c8d620f [Mohammad Arshad] ZOOKEEPER-2307:ZooKeeper not starting because acceptedEpoch is less than the currentEpoch

(cherry picked from commit 19d8567)

[symat]

@anmolnar , @maoling , this is a fix you already reviewed (#1145), and I was merging it to master and branch-3.5 back then, but it wasn't merged to branch-3.6 for some reason (this happened around the branching of RC1 of 3.6.0). See ZOOKEEPER-4222.

Please take a look. I think we should submit this to the current branch-3.6 if CI is OK with it.

[symat]

CI failed, we hit one timeout in test ReadOnlyModeTest. My local re-run succeeded ( mvn test -Dtest=ReadOnlyModeTest -DfailIfNoTests=false), most probably this is only an infra / flaky CI issue. Given that this was a clean cherry-pick, I think it is OK to go ahead and merge it.

[nkalmar]

+1, same changes as on master/3.5

[symat]

thanks @nkalmar !
I'll merge this (after lunch... let's keep the priorities :p :) )

[anmolnar]

lgtm. I restarted the CI build.

[symat]

thanks @anmolnar !
it succeeded this time