Abstract leveldown v4

[ralphtheninja]

Just a temporary branch and a temporary PR to track build status. This PR should never be merged.

Actually, why not just use this PR for updating to abstract-leveldown@4.0.0. It doesn't matter if this PR contains temporary commits, since we can just squash the commits when we're done.

[ralphtheninja]

Current abstract-leveldown#master should be building fine. Once confirmed, I'll switch to abstract-leveldown#close-dbs.

[ralphtheninja]

Switching to abstract-leveldown#close-dbs should fail on Windows.

[ralphtheninja]

@vweevers Seems windows builds are passing now :)

[ralphtheninja]

Will keep this PR open for a while and do amend on the latest commit as soon as master changes for abstract-leveldown so we can re-trigger builds on travis and appveyor.

[ralphtheninja]

@vweevers Interesting. Got some weird error on AppVeyor I haven't seen before

https://ci.appveyor.com/project/Level/leveldown/build/258/job/q90jmulx538af4yg

[ralphtheninja]

Travis has been acting really weird the past days/week. This time it failed to download node 5 binaries with nvm. Restarted that build.

[ralphtheninja]

Travis builds for OSX seem to be borked and have been quite some time.

[vweevers]

@ralphtheninja do you intend to squash everything into one commit? Because I think there's enough here to warrant separate commits.

[ralphtheninja]

@vweevers Yes, will squash.

[ralphtheninja]

@vweevers I could make a new branch and cherry pick the stuff from this branch that are essential.

[vweevers]

Or rebase to remove the temporary commits, whatever works.

In any case, the end result LGTM.

[ralphtheninja]

Should we release this as a patch?

[vweevers]

Not sure. However small it may be, Level/abstract-leveldown@a2621ad is a breaking change, inherited by implementations.

[ralphtheninja]

Then it should be a major.