Update magic number from 2 to 3.

[deepakjois]

Fixes #282.

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This change is 

[deepakjois]

We should wait to merge this. While we are waiting, I would like to better understand the implications of this for stuff that is currently using Badger, and the guidance we can offer.

So if I understand correctly, any data that was written using a previous build of Badger cannot be read once this change goes in. One hacky workaround is to manually rewrite the Badger version from 2 -> 3 in the manifest file. Even then, it would only work if the data was written using a build of Badger post v0.8, because we made some changes to the way data was written on disk.

[manishrjain]

We need to provide a way to backup Badger, to a standard key-value file; possibly using protocol buffers for extensibility. This needs to go into both v0.8 and v1.0 -- see BoltDB for reference. Once we have that, we need an import tool which can rebuild Badger.

[magik6k]

Just a note - we are using badger with this patch applied in IPFS 0.4.12-rc1, see https://github.com/gxed/badger/commits/master.

[deepakjois]

@magik6k This is now in master, but there are some other changes as well (see #305 ). We decided to get those in because we would have needed to change the manifest version for those change.

The changes in #305 also change the Txn.Set() API slightly.