* We had a usage of `Observable.subscribe` that uses the deprecated signature with 3 arguments. These changes switch to the non-deprecated version that passes in an `Observer`.
* Avoids always creating a `complete` callback since it isn't required.
* We were repeating all of the internal callbacks twice: once for sync and once for async. These changes move them out into variables so that they're more minifier-friendly. The savings aren't huge (~100 bytes minified), but it doesn't add any maintenance effort on our end so I decided to add it.
PR Close#41450
d641542587