Resolve Deadlock: Allow Concurrent Creation of Non-First Index on AO

[ashwinstar]

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AlterTableCreateAoBlkdirTable() is being called unconditionally, it upgrades ShareLock to ShareRowExclusiveLock hence may result in deadlock for concurrent index creation even if block directory is present on the append-optimized table. This is regression in 7X compared to 6X. As a fix avoiding call to AlterTableCreateAoBlkdirTable() if block directory is present which is checked already in DefineIndex() with ShareLock held.

Just note: the deadlock case for concurrent index creations in absence of block directory (which is initial index) still exists, as in absence of block directory, lock upgrade from ShareLock to ShareRowExclusiveLock in DefineIndex() still exists and is hard to resolve. Existing test concurrent_index_creation_should_not_deadlock wasn't testing the case correctly. Hence modifying that test to only validate for now non-first index creation case.

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