sql: schema change repeatedly retries with gcttl error about cockroach

[blathers-crl]

Hi @itsbilal, please add branch-* labels to identify which branch(es) this C-bug affects.

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[fqazi]

We are running into two problems, in this scenario:

1. We always clear the protected timestamp even if a retryable error is hit, see:

   cockroach/pkg/sql/index_backfiller.go

   Line 86 in c5522ce

   defer func() {

2. The readAsOf timestamp does not properly take into account the current time, if a retry happens it will assume GC TTL * 0.8 time has to pass again:

   cockroach/pkg/jobs/jobsprotectedts/jobs_protected_ts_manager.go

   Line 129 in c5522ce

   waitBeforeProtectedTS := time.Duration((time.Duration(zoneCfg.GC.TTLSeconds) * time.Second).Seconds() *