planner: the optimizer can't use the latest tidb_mem_quota_binding_cache value to initialize binding cache #65381

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Step 1: create massive bindings, and their total memory usage is larger than the default binding cache size 64MB
Step 2: restart TiDB and we'll see errors like he memory usage of all available bindings exceeds the cache's mem quota.
Step 3: increase the binding cache size through set global tidb_mem_quota_binding_cache = 1073741824;
Step 4: restart TiDB again, there should be no binding cache errors.

2. What did you expect to see? (Required)

In Step 4, there are still some binding cache errors:

The root cause is that when starting TiDB nodes, we initialize binding cache before initializing system variables, so the optimizer can't get the latest value of tidb_mem_quota_binding_cache and then use the default value 64MB instead.

3. What did you see instead (Required)

No error

4. What is your TiDB version? (Required)

v7.5, v8.5, Master