RawKV CompareAndSwap should support setting a TTL

[jcjones]

The CompareAndSwap method doesn't have an ability to take as an input the TTL of the resulting Key/Value, however the underlying kvrpcpb protobuf does have an (unused) entry for TTL, and the value is read and used by TiKV.

Request:
Add support to set a TTL during a CAS.

Use case:
When one uses CAS to create a new KV without overwriting an existing value, it's nice to be able to include a TTL in the initial setting, rather than having to do shenanigans that amount to a follow-on Put with a TTL.