Optimize couch_util:reorder_results/2,3

This function is used in the hot path of _revs_diff and _bulk_docs API calls.
Those could always use a bit more optimization:

  * In `_revs_diff` it's used when fetching all the FDIs to see which docs are
    missing in `couch_btree:lookup/2`.

  * In `_bulk_docs` it's used in the `fabric_doc_update` when finalizing the
    response.

Using erlperf in #4051 noticed an at most 5x speedup from using a map instead
of a dict. Since a map already falls back to a proplist for small sizes, skip
the length guard.

Some erlperf examples from #4051:

500 Keys
```
> f(Keys), f(Res), {Keys, Res} = Gen(500), ok.

> erlperf:run(#{runner => {couch_util, reorder_results2, [Keys, Res, 100, dict]}}).
2407
> erlperf:run(#{runner => {couch_util, reorder_results2, [Keys, Res, 100, map]}}).
11639
```

Using a map without the guard, which is the change in this this PR:

```
> f(Keys), f(Res), {Keys, Res} = Gen(500), ok.
ok

> erlperf:run(#{runner => {couch_util, reorder_results, [Keys, Res]}}).
12395
> erlperf:run(#{runner => {couch_util, reorder_results, [Keys, Res]}}).
12508
```

As a bonus this also cleans up the code a bit, too.

src/couch/src/couch_util.erl

@@ -523,24 +523,13 @@ verify(X, Y) when is_list(X) and is_list(Y) ->
 verify(_X, _Y) ->
  false.
 
-% linear search is faster for small lists, length() is 0.5 ms for 100k list
-reorder_results(Keys, SortedResults) when length(Keys) < 100 ->
- [couch_util:get_value(Key, SortedResults) || Key <- Keys];
 reorder_results(Keys, SortedResults) ->
- KeyDict = dict:from_list(SortedResults),
- [dict:fetch(Key, KeyDict) || Key <- Keys].
+ Map = maps:from_list(SortedResults),
+ [maps:get(Key, Map) || Key <- Keys].
 
-reorder_results(Keys, SortedResults, Default) when length(Keys) < 100 ->
- [couch_util:get_value(Key, SortedResults, Default) || Key <- Keys];
 reorder_results(Keys, SortedResults, Default) ->
- KeyDict = dict:from_list(SortedResults),
- DefaultFunc = fun({Key, Dict}) ->
- case dict:is_key(Key, Dict) of
- true -> dict:fetch(Key, Dict);
- false -> Default
- end
- end,
- [DefaultFunc({Key, KeyDict}) || Key <- Keys].
+ Map = maps:from_list(SortedResults),
+ [maps:get(Key, Map, Default) || Key <- Keys].
 
 url_strip_password(Url) ->
  re:replace(