Add mixins for using polling in data services

[mcmire]

Explanation

The polling-controller package contains mixins for integrating the polling pattern into a class. Using these mixins on their own is somewhat cumbersome, so the package also contains utility functions which can be used in place of a superclass.

Currently, there are two styles of polling supported — one that subscribes to the block tracker, the other that runs on a cadence — and then past this, there are variants of these utility functions for controllers and non-controllers.

However, there is no variant for data services. That means if engineers want to integrate polling into data services, they are out of luck. This commit corrects that by adding two new mixins: BlockTrackerPollingDataService and StaticIntervalPollingDataService.

References

Closes https://consensyssoftware.atlassian.net/browse/WPC-1038.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

[mcmire]

I thought about placing these in base-data-service instead of this package, since we want to encourage engineers to use polling in data services and discourage them from using it in controllers. But then base-data-service would depend on polling-controller, and that seemed a little strange.