feat(query): Improve query performance

As a follow on to #17596, performance of all merge operations can
be improved by removing allocations when comparing tags.

`benchstat` results:

```
name                    old time/op    new time/op    delta
SortedMergeIterator-16    32.4ms ± 2%     5.2ms ± 3%  -83.81%  (p=0.000 n=10+10)

name                    old alloc/op   new alloc/op   delta
SortedMergeIterator-16    36.5MB ± 0%     5.8MB ± 0%  -84.20%  (p=0.000 n=10+9)

name                    old allocs/op  new allocs/op  delta
SortedMergeIterator-16      420k ± 0%       60k ± 0%  -85.71%  (p=0.000 n=9+10)
```