Increase API parity with `levelup`

[vweevers]

Biting the bullet. This adds promise support, makes open() & close() idempotent, adds open and close events, makes an abstract-leveldown db safer than a levelup db, and more.

Needs a ton of (canary) tests. Contains a few subtle breaking changes, but for consumers of downstream implementations it should be painless. An abstract-leveldown db can still be wrapped with levelup and folks can continue to use that interface.

I'm opening this up for early review, I might take it even further.

Ref https://github.com/Level/community/issues/58 Ref https://github.com/Level/leveldown/issues/8

[vweevers]

Complexity is slowly increasing here and I'm wary of that. So far it's still worth it. I find the behavior easier to oversee, and reimplementing certain levelup features (like idempotent open) gives me a chance to polish them and remove the need for safety checks that we've had to add to leveldown.

I've been canary testing this against various modules; subleveldown in particular because I sense that this module will tell me when I've found a balance between increased abstract-leveldown complexity and reduced subleveldown complexity.

[vweevers]

Funny how this is starting to look like the levelup from 8 years ago.

An abstract-leveldown db can still be wrapped with levelup and folks can continue to use that interface.

Technically that still works (with a few tweaks in levelup) but there's no value in doing that anymore (except if you need streams, but that will be easy to move to a module). Thinking about forking abstract-leveldown including dependents, for a clean break. For now I'm continuing here, because I have no reason to publish it yet and I've yet to prove this is all worth it. Next up are encodings and maybe a light form of hooks.

[vweevers]

Moved to https://github.com/Level/abstract-level.