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.

frontend_tests/zjsunit/zblueslip.js

@@ -119,7 +119,9 @@ exports.make_zblueslip = function () {
         return ex.message;
     };
 
-    lib.start_timing = () => () => {};
+    lib.measure_time = (label, f) => {
+        f();
+    };
 
     lib.preview_node = (node) => "node:" + node;