4.3.0 pre-release, beta 6 = Fork Release for block 1 200 000

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If you're late upgrading, please use
https://github.com/bismuthfoundation/Bismuth/releases/tag/v4.3.0.1-beta.1

This was the most up to date candidate for 4.3.0 release, now considered as stable.

4.3.0 will be a mandatory update, in order to support the upcoming fork at block 1 200 000.
That will be around june 6.
See: https://hypernodes.bismuth.live/?p=586

This pre-release passed internal testing, it's likely to become the final release.

Service operators, pools, exchanges, are invited to test and upgrade to that version.

Change log from previous beta: slight adjustments to allowed versions to avoid network partitions, better compatibility with previous config files, fix to plugin api (block, fullblock and rollback events)