Used a ThreadPoolExecutor to make concurrent API calls

[arueth]

Fixes #353

Background

Currently the fronted makes serial calls to the backend services, with this change it will make concurrent calls and therefore return the page quicker.

Change Summary

Change from serial to concurrent calls to the backend services.

Testing Procedure

Deploy the application in an environment and check the loading times for the home page.

My development environment is here: http://34.168.42.167/

Related PRs or Issues

None

[bourgeoisor]

@arueth looks like all checks are passing-- I tried out your live demo, it seems fairly snappy. Could you check Cloud Tracing and post a screenshot showing the calls happening in parallel? (see the description of #353 for the non-parallel screenshot)

[arueth]

The trace doesn't seem to look like that anymore. I'm not sure if it the product of the change or a previous change.

[bourgeoisor]

This is what I get on the current latest release (https://bank-of-anthos.xyz):

So it seems that's this PR's changes

[arueth]

Using the threads must not play nice with the open telemetry plugin.

[arueth]

I'm doing some refactoring to get the tracing working, expect an update shortly.

[arueth]

becomes

[bourgeoisor]

LGTM!