Add sync execution methods and performance optimization.

[snowyu]

• Add the OpenSync, GetSync, PutSync, DeleteSync, BatchSync, Iterator.NextSync, Iterator.EndSync
• Optimize the iterator performance and memory-leak fixed
• Add the bench/bench.js to compare
• Using the new snowyu/abstract-leveldown#feature/sync to test

[Edit: the link to the discussion in abstract-leveldon is https://github.com/Level/abstract-leveldown/pull/49 - @mcollina]

[ralphtheninja]

A couple of things:

• What is the reason for all these synchronous methods?
• This is a quite a big PR. Is there any way you could split it up into smaller changes so they can be discussed more easily? Updating leveldb version should really be a separate PR imo.
• I noticed some commits were erroneous (the ones related to Travis). It would be nice if you could prune them.

[mcollina]

👍 on separate PRs.

I really like the openSync method, I understand why you might want to provide getSync and putSync, but I am missing the point for the iterator. It's as fast as the non-sync one, and it just adds code to maintain.

[snowyu]

in fact , I just wanna the OpenSync, and GetSync are enough for me.

iterator sync just as compare test :).

[snowyu]

But leave these choose to others, not me to choose for them.

[snowyu]

@ralphtheninja: already rebased.

I've publish it as leveldown-sync for my requirement. also levelup-sync and memdown-sync made.

[ralphtheninja]

This would have to be re-implemented since we're now based on n-api.