MDEV-22794: Avoid potential rollback segment contention with

            increased scalability through even distribution

Rollback segments are allocated to transactions in round-robin fashion.
This is controlled by incrementing a static-scope counter named rseg_slot.

Said logic is not protected by any mutex or use of atomic for the counter.
This potentially can cause the same rollback segment to get allocated to
N different transactions (requesting allocation at the same time).
While this is not an issue as a rollback segment can host multiple
transactions from contention (performance) perspective it is better to
allocate these rollback segments in round-robin fashion.

Fix for the said issue ports use of atomic for the said counter that would
ensure the original design semantic (even distribution through round-robin)
is retained.

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