Use GC-TTL to keep the GC Lifetime

[kennytm]

Feature Request

Is your feature request related to a problem? Please describe:

Currently we use UPDATE mysql.tidb SET VARIABLE_VALUE = '100h' WHERE VARIABLE_NAME = 'tikv_gc_life_time' to disable the GC. This is error-prone when Lightning exits before this can be reverted.

Describe the feature you'd like:

Use the more recent GC-TTL API so that PD can automatically re-enable GC even when Lightning exits accidentally.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Optimization:

[kennytm]

Note: GC-TTL should only be used when the target cluster is 4.0.0+. On 3.x, keep using the old method.