Systematic fix for allocations on read-only MEM_ROOT after first execution

After the first execution of a prepared statement or stored routine, its MEM_ROOT is marked read-only (MY_ROOT_READ_ONLY, flag 4) so that later executions cannot allocate persistent memory on it. Few code paths still try to allocate lazily on such MEM_ROOT after first execution, triggering the assert (mem_root->flags & 4) == 0.

Ex:

CREATE PROCEDURE p(x INT DEFAULT (SELECT 1)) ...

CALL p(1); -- first exec, marks mem_root read-only

CALL p(); -- tries to allocate on locked mem_root when executing DEFAULT

The current workaround is to clear the flag temporarily, allocate and again mark the mem_root read-only. This bypasses the protection and need a better strategy to handle such allocations.