kv: combine heap allocations in maybeStripInFlightWrites

This commit combines two of the heap allocations incurred by 1PC calls
to `maybeStripInFlightWrites` when making a shallow copy of the provided
`BatchRequest` into a single allocation.

In a write-heavy workload, these were observed to account for about **0.8%** of
all heap allocations, meaning that this change should reduce heap allocations in
that workload by about **0.4%**.

```
      File: cockroach
Type: alloc_objects
Time: Dec 31, 2021 at 3:51am (UTC)
Active filters:
   focus=maybeStripInFlightWrites
Showing nodes accounting for 2259283, 1.37% of 164666499 total
----------------------------------------------------------+-------------
      flat  flat%   sum%        cum   cum%   calls calls% + context
----------------------------------------------------------+-------------
...
----------------------------------------------------------+-------------
                                            506152   100% |   github.com/cockroachdb/cockroach/pkg/kv/kvserver.(*Replica).sendWithRangeID /go/src/github.com/cockroachdb/cockroach/pkg/kv/kvserver/replica_send.go:140
         0     0%  0.63%     506152  0.31%                | github.com/cockroachdb/cockroach/pkg/kv/kvserver.maybeStripInFlightWrites /go/src/github.com/cockroachdb/cockroach/pkg/kv/kvserver/replica_batch_updates.go:58
                                            506152   100% |   github.com/cockroachdb/cockroach/pkg/roachpb.(*EndTxnRequest).ShallowCopy /go/src/github.com/cockroachdb/cockroach/pkg/roachpb/api.go:797 (inline)
----------------------------------------------------------+-------------
                                            720901   100% |   github.com/cockroachdb/cockroach/pkg/kv/kvserver.(*Replica).sendWithRangeID /go/src/github.com/cockroachdb/cockroach/pkg/kv/kvserver/replica_send.go:140
         0     0%  0.63%     720901  0.44%                | github.com/cockroachdb/cockroach/pkg/kv/kvserver.maybeStripInFlightWrites /go/src/github.com/cockroachdb/cockroach/pkg/kv/kvserver/replica_batch_updates.go:62
                                            720901   100% |   github.com/cockroachdb/cockroach/pkg/roachpb.(*RequestUnion).MustSetInner /go/src/github.com/cockroachdb/cockroach/pkg/roachpb/batch_generated.go:385
----------------------------------------------------------+-------------
```

pkg/kv/kvserver/replica_batch_updates.go

@@ -55,11 +55,17 @@ func maybeStripInFlightWrites(ba *roachpb.BatchRequest) (*roachpb.BatchRequest,
 	// append. Code below can use origET to recreate the in-flight write set if
 	// any elements remain in it.
 	origET := et
-	et = origET.ShallowCopy().(*roachpb.EndTxnRequest)
+	etAlloc := new(struct {
+		et    roachpb.EndTxnRequest
+		union roachpb.RequestUnion_EndTxn
+	})
+	etAlloc.et = *origET // shallow copy
+	etAlloc.union.EndTxn = &etAlloc.et
+	et = &etAlloc.et
 	et.InFlightWrites = nil
 	et.LockSpans = et.LockSpans[:len(et.LockSpans):len(et.LockSpans)] // immutable
 	ba.Requests = append([]roachpb.RequestUnion(nil), ba.Requests...)
-	ba.Requests[len(ba.Requests)-1].MustSetInner(et)
+	ba.Requests[len(ba.Requests)-1].Value = &etAlloc.union
 
 	// Fast-path: If we know that this batch contains all of the transaction's
 	// in-flight writes, then we can avoid searching in the in-flight writes set