Creating Materialized views concurrently leads to missing data

nutbunnies was writing dtests that create multiple tables concurrently. He also wrote a test that creates multiple MV but has not been able to get it works properly. After some debugging outside of dtest, it seems that there is an issue if we create more than 1 MV at the same time. There is no errors in the log but the MV are never entirely populated and are missing data.

I've attached 2 scripts:

mv_test_bad.sh: is the one that reproduce the issue. It creates 4 MVs at the same time. At the end, some data are missing in the MVs and there is nothing in system.hints or system.batchlog.

mv_test_good.sh: is the same script but that waits 10 seconds between each MV creation, which results in 4 MVs with all the data.

Some more notes from Andrew:

- lowering the number of rows inserted below ~1000 won't exhibit the inconsistent behavior
- adding more columns/MV make it worse -- more of the MVs counts are consistently wrong
- multiple runs will range in disagreement -- usually one of the MVs is correct though
- the describe cluster and system.mv* queries always "look" good

Thanks Andrew for finding this bug and providing the test scripts!

//cc carlyeks tjake enigmacurry