Cargo.toml: set version to 3.0.9 and update TiKV deps to 3.0.8

[kennytm]

*: removed some unused functions

What have you changed? (mandatory)

set version to 3.0.9

What are the type of the changes? (mandatory)

• Engineering (engineering change which doesn't change any feature or fix any issue)

How has this PR been tested? (mandatory)

cargo test

Does this PR affect TiDB Lightning? (mandatory)

No

Refer to a related PR or issue link (optional)

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[kennytm]

PTAL @WangXiangUSTC @3pointer

[WangXiangUSTC]

LGTM