Add Typescript type definitions for most of Racer's Model API#2

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ericyhwang wants to merge 4 commits intomainfrom typescript

+1,116-14

Commits on Jun 20, 2019

Add some TypeScript-compatible JSDoc for util.js
ericyhwang
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Commits on Jun 21, 2019

Commits on Jun 28, 2019

TypeScript definitions: Have RacerBackend extend sharedb.Backend, to match JS implementation
ericyhwang
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