Improve the scalar chart user experience

[wchargin]

Summary:
This fixes a number of issues with the scalar charts, some of which are
interdependent, and all of which are existing issues made more visible
by the presence of a search pane.

In particular:

1. If a chart's run and tag properties are changed simultaneously,
   two updates will be triggered, and the first update will see the
   data in an inconsistent state (e.g., Tag B but still Run A). We fix
   the resulting issues by making observer updates asynchronous, so
   that they only actually take effect once all changes have flushed.
   (This fixes Run-tag updates are not atomic #169.)

2. To reduce latency due to the above solution, we further debounce
   these update handlers.

3. Because the handlers are asynchronous and debounced, we need to
   maintain stronger invariants on the caching keys. (In particular, if
   a chart changes from Tag A to Tag B and then to Tag C, we don't want
   the network requests for Tag B to fill the runs-loaded cache and
   then have Tag C forgo its fetches.)

4. Furthermore, when the tag of a chart is changed, we display a
   loading indicator. This is to prevent problems in the scenario where
   many charts change from one tag to another: in this case, all the
   charts' titles will update immediately (they're just bound to text
   nodes in the DOM), but charts' data will take a while to update; the
   result is that there are many charts with titles that simply do not
   match their data, which is at best very confusing. The loading
   indicator makes it clear that the data is stale.

5. When a chart's tag is changed, we now reset the domain of the chart
   once all the data has loaded. (This fixes Race condition (consistent repro): wherein chart domain is set only after data arrives #164.)

6. Finally, we offer a button to manually reset the domain of the
   chart. (This is often useful when changing the set of selected
   runs: after adding a new run, you may need to expand the domain, but
   we don't do this automatically because you may often want to keep
   your old domain.)

Issues (1), (4), and (5) arise frequently when changing a search query,
as this causes many charts to change their tag and runs.

Issues (1), (2), and (4) apply to other dashboards as well, but have not
been fixed. (Issue (3) would apply to other dashboards that display one
visualization per tag instead of per run-tag combination, but there are
no such dashboards.)

Test Plan:
Cherry-pick #173 before proceeding. (Without a search feature, the
effects of this change are minimal---but I want to merge this change
before I merge the search feature!)

Launch TensorBoard with a few dozen scalar tags. Suppose without loss of
generality that the name of the fifth tag displayed in the search
results pane (on query .*) is foo. Set the search query to
(?!foo); this should still match all tags. Now, add a caret at the
beginning to search for ^(?!foo). Charts past the fifth one should
reload. Check that the reloading behavior is pleasant.

Then, zoom into some charts, manipulating their windows. Click the
"reload" button, or wait 30 seconds, and ensure that the window is
preserved. Change the search query, though, and the windows should be
reset to show all the data.

wchargin-branch: improve-scalars-ux