[WT-2535] Extend test/format to test for transactions reading their writes - MongoDB

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In ~~WT-2534~~ we spent a lot of time adding diagnostics to track down a bug found when porting to a system with a weaker memory model than x86.

It would be helpful to have the following simple test for lost updates, which I believe would have failed on the test system:

create a table with one record, value_format=q, initial value zero

In N threads:

  count = 0

  while count < 1000:

    begin a transaction with isolation=snapshot

    read $value

    update with $value+1, rollback and continue on failure

    commit

    increment count

Join all threads, verify that the value is 1000 * N.