r/group_manager: protect against race with set_ready()

[ztlpn]

set_ready() may race with group creation if it is called after a consensus instance is created, but before it is added to the _groups collection.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x
• v23.1.x

Release Notes

Bug Fixes

• Protect against a very rare scenario where after node restart, some of the partition replicas hosted on that node could not take part in leader elections.

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/43671#018cf8d6-75f9-41ba-9a1a-2ef1ec00e9c1

[vbotbuildovich]

/backport v23.3.x

[vbotbuildovich]

/backport v23.2.x

[vbotbuildovich]

/backport v23.1.x

[vbotbuildovich]

Failed to create a backport PR to v23.1.x branch. I tried:

git remote add upstream https://github.com/redpanda-data/redpanda.git
git fetch --all
git checkout -b backport-pr-16068-v23.1.x-842 remotes/upstream/v23.1.x
git cherry-pick -x 8139e1ed1408289a70b6a50487efb4a8155570ba

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