util/interval/generic: avoid new allocations in btree benchmarks

The generic template for interval btree makes an attempt to work with
both value and pointer parameterized types. To facilitate this option
in tests, it used reflection to create new instances of the item type.
This reflection was preventing allocations of the item type from being
inlined and avoided by escape analysis, which was skewing benchmarks.

This commit fixes this by avoiding the reflection and instead adding
a constructor to the contract of the parameterized type. This is cleaner
and allows the allocations to be avoided in these benchmarks.

pkg/storage/spanlatch/manager.go

@@ -101,6 +101,7 @@ func (la *latch) ID() uint64         { return la.id }
 func (la *latch) Key() []byte        { return la.span.Key }
 func (la *latch) EndKey() []byte     { return la.span.EndKey }
 func (la *latch) String() string     { return fmt.Sprintf("%s@%s", la.span, la.ts) }
+func (la *latch) New() *latch        { return new(latch) }
 func (la *latch) SetID(v uint64)     { la.id = v }
 func (la *latch) SetKey(v []byte)    { la.span.Key = v }
 func (la *latch) SetEndKey(v []byte) { la.span.EndKey = v }