kvclient: figure out the MVCC timestamp for the buffered write

Currently, if the write is served from the buffer, it'll come back with an empty MVCC timestamp
```sql
create table t (k int primary key);
set kv_transaction_buffered_writes_enabled = true;
begin;
insert into t values (1);
-- returns 0.0000000000
select crdb_internal_mvcc_timestamp from t;
commit;
-- returns write timestamp
select crdb_internal_mvcc_timestamp from t;
```
We probably should synthesize the MVCC timestamp at _some_ point (either when adding to the buffer or when serving a read from the buffer). We should figure out where that happens on the KV server side and ensure that we don't break any invariants there.

Jira issue: CRDB-49263

Start with the SQL reproduction using buffered writes, then trace the KV server-side path for writes served from the buffer. Determine whether the MVCC timestamp should be synthesized when buffering or when serving the read, while checking the relevant invariants. Done means the buffered read returns a correct MVCC timestamp without changing the post-commit timestamp behavior.

Written by the indexing model from the issue text.