`tidb_constraint_check_in_place_pessimistic` should not be global scope, and should not affect internal sessions

[ekexium]

Enhancement

Changing this variable can lead to unexpected results. Internals sessions shouldn't be affected by it as well, e.g. the imeplementation of get_lock() depends on tidb_contraint_check_in_place_pessimistic=1to work.
We should only allow sessions that are aware of its effects use it.

[sticnarf]

Is it the exact reason of the inconsistency in system tables?

[ekexium]

Is it the exact reason of the inconsistency in system tables?

Well I haven't investigated it. It might be. After the fix we can see if it still happens since it is said to be reproducible.