MDEV-39265: Assertion (mem_root->flags & 4) == 0 failed upon 2nd execution USING DEFAULT with sequence

[raghunandanbhat]

fixes MDEV-39265

Problem:

When a prepared statement uses DEFAULT with sequence, on its second or subsequent execution, server tries to allocate TABLE_LIST object (used for adding sequence table to internal_tables list) on statement's memory root. This fails with an assert because statement's memory root is marked read-only after the first execution.

  CREATE SEQUENCE s;
  CREATE TABLE t (a INT DEFAULT(NEXTVAL(s)));
  PREPARE stmt FROM "UPDATE t SET a = ?";

  EXECUTE stmt USING 3; -- first execution, marks mem_root read-only
  EXECUTE stmt USING DEFAULT;  -- tries to allocate on second execution

Fix:

Temporarily clear the read-only flag on the statement memory root to allow the allocation. Also adds a debug hook to verify allocation happens not more than once per prepared statement and per internal table added to the list.