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trx->read_view|= 1 was done in a silly attempt to fix race condition
where trx->read_view was closed without trx_sys.mutex lock by read-only
trasnactions.

This just made the problem less likely to happen. In fact there was race
condition in const version of trx_get_read_view(): pointer may change to
garbage any moment after MVCC::is_view_active(trx->read_view) check and
before this function returns.

This patch doesn't fix this race condition, but rather makes it's
consequences less destructive.