Breaking: remove callbacks in favor of promises

[vweevers]

Applies to both the public and private API. If you were already using promises in the public API, nothing changed, except for subtle timing differences. In the private API, it means that function signatures like _get(key, options, callback) have changed to async _get(key, options).

In addition:

• Remove the deprecated iterator.end() alias of .close() because we'd otherwise have to keep callback support on this one method.
• Replace use of process.nextTick() with queueMicrotask() because there isn't a better time to introduce timing differences
• Lay the groundwork for supporting AbortSignal on iterators (not functional yet). I'll probably implement (and explain) this in a follow-up PR.

TODO:

• [x] Update tests
• [x] Fix remaining tests (mainly bugs in opening & closing)
• [x] Check browser & Node.js support of JS features (like signals)
• [x] Canary-test on memory-level
• [x] Canary-test on classic-level (just 2 or 3 methods)
• [x] Benchmark (later)

[vweevers]

Ouch: I wondered why tests are passing in Node 12 (I didn't expect it to, for lack of AbortController). Turns out, they aren't passing, but the exit code is 0. Must be a bug in faucet which was recently updated.

[vweevers]

Now comes the fun part: using promises in the C++ of classic-level.

Edit: that's surprisingly easy (at least for basic methods like put()) and performance is roughly the same. No blockers :)