Add db.getMany(keys)

[vweevers]

Tested against leveldown, memdown, level-js, deferred-leveldown, encoding-down, (indirectly) levelup, subleveldown.

Ref Level/community#101

[vweevers]

Note that here we're doing everything that levelup normally does, as an experiment.

[vweevers]

Which means that levelup only has to do this:

LevelUP.prototype.getMany = function (keys, options, callback) {
  return this.db.getMany(keys, options, callback)
}

[ralphtheninja]

So later on we can move over the catering stuff to abstract-leveldown one by one.

[vweevers]

Added in the last commit (1f1ab97):

• AbstractLevelDOWN.prototype.isOperational() which returns true if the db accepts operations. Normally that means when db.status is 'open', and deferred-leveldown can override _isOperational() to also accept operations when db.status is 'opening'.
• More tests

And removed:

• test custom _serialize* tests which were not that valuable and not monkeypatch-safe. These tests were failing in deferred-leveldown.