The old blob implementation had code that attempted to upgrade the lock mode for
LM_CommittedRead operations, but it did not work properly as it did not recompute
the operation flags.

As a consequence, reading a blob with LM_CommittedRead could return inconsistent
data, with different part of the read data being from different commits done by
other transactions.

The fix is to correctly recompute all necessary flags when upgrading lock mode.