Update SharedLocalSnapshot correctly for subtransactions
QE reader leverages SharedLocalSnapshot to perform visibility checks. QE writer is responsible to keep the SharedLocalSnapshot up to date. Before this fix, SharedLocalSnapshot was only updated by writer while acquiring the snapshot. But if transaction id is assigned to subtransaction after it has taken the snapshot, it was not reflected. Due to this when QE reader called TransactionIdIsCurrentTransactionId, it may get sometimes false based on timings for subtransaction ids used by QE writer to insert/update tuples. Hence to fix the situation, SharedLocalSnapshot is now updated when assigning transaction id and deregistered if subtransaction aborts. Also, adding faultinjector to suspend cursor QE reader instead of guc/sleep used in past. Moving cursor tests from bugbuster to ICG and adding deterministic test to exercise the behavior. Fixes #1276, reported by @pengzhout
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