async func for CausalTsProvider

[zeminzhou]

Feature Request

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

Sometime we need get causal ts or flush asynchronously. When we call flush to flush ts in a async func (such as register_min_ts_event), the async flush can get better performance.

Describe the feature you'd like:

Add async func for CausalTsProvider.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: