tso: use OrderedSingleFlight to reduce data race on logical overflow update

## Enhancement Task

### Problem

In `timestampOracle.getTS`, when the TSO logical part overflows (`overflowedLogical`),
multiple concurrent goroutines can simultaneously call `updateTimestamp(overflowUpdate)`.
This causes redundant concurrent writes to the in-memory TSO state (`tsoMux`) and
unnecessary repeated etcd writes, and can trigger data races under the race detector.

### Solution

Wrap the `updateTimestamp(overflowUpdate)` call in `getTS` with `OrderedSingleFlight.Do`,
so that concurrent overflow-triggered updates are coalesced into a single execution.
The ordered guarantee ensures each caller gets a result from an execution that began
after its own arrival, preventing stale-result issues.

### Affected Files

- `pkg/tso/tso.go`
- `pkg/tso/allocator.go`