blueslip: Add measure_time wrapper.

Now when we want to measure how long a block
of code takes to execute, we just wrap it with
`blueslip.measure_time`, instead of the awkward
idiom from my original commit of getting a callback
function.

My rationale for the original scheme was that I
wanted to minimize diffs and avoid changing
`const` to `let` in a few cases, but I believe
now that the function wrapper is nicer.

In a few cases I just removed the blueslip timing
code, since I was able to confirm on czo that
the times were pretty minimal.

static/js/list_widget.js

@@ -186,7 +186,6 @@ exports.create = function ($container, list, opts) {
 
         const slice = meta.filtered_list.slice(meta.offset, meta.offset + load_count);
 
-        const finish = blueslip.start_timing("ListWidget " + opts.name);
         let html = "";
         for (const item of slice) {
             const s = opts.modifier(item);
@@ -202,8 +201,6 @@ exports.create = function ($container, list, opts) {
             }
         }
 
-        finish();
-
         $container.append($(html));
         meta.offset += load_count;
     };