Refactor open() and close() to mostly bypass levelup

[vweevers]

Closes #60, closes #77, closes #78, and opens up the possibility to add a manifest.

I was sitting around thinking "how can i make subleveldown more complicated" and this PR is the result.

[vweevers]

To simplify, maybe we should say that a user must manage the open state of a db outside of subleveldown. Meaning:

• You must pass in either an opening or open db, else we throw
• Closing a sublevel is a noop. To me it feels wrong for a sublevel to close its "parent" db.
• It should however be valid to reopen a closed sublevel - but it should not touch anything except subleveldown state.

[ralphtheninja]

I was sitting around thinking "how can i make subleveldown more complicated" and this PR is the result.

😆

[vweevers]

Alternative strategy:

• Only accept a levelup db as input (breaking change), so that we can hook into open events
• Check that by if (db.supports.deferredOpen) so we can support abstract-leveldown db's later on
• Make that if (db.supports.deferredOpen || reachdown.is(db, 'levelup') for backward compat
• Never open or close the db. Instead merely wait for it.

[vweevers]

In other words, the input db must open itself (or once closed by the user, be explicitly reopened by the user) because sublevels shouldn't concern themselves with (the state of) the rest of the db.

Closing this PR, I don't like it. Will explore the alternative strategy.