Clean up async usage to make code more readable/maintainable

[arthurhsu]

Use async await function to make code more readable/maintainable, for example:

https://github.com/arthurhsu/lovefield-ts/commit/99e02aa2ae96f8545e542f8861b9ad495832b853#diff-704b87d3b3a26b83ec3ba9434da2a54bL55

However, Lovefield uses a lot of parallelization techniques, so caveats pointed out in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function needs to be kept in mind.