Experiment: serializable topo

[shlomi-noach]

Description

An experiment to see whether endtoend topo-related flakiness is resolved by setting etcd topo as serializable. An alternate tale on #13781, so instead of running "wait-for" loops we might just be able to get a consistent read from topo.

Testing: run backup_pitr a bunch of times, see if flakiness is still there.

Related Issue(s)

Checklist

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[vitess-bot]

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[shlomi-noach]

Closing as solution implemented in #13791