Anonymous IPFS

We've received many requests for anonymity support in go-ipfs. This issue tracks the issue and the related concerns.

First, there was a significant amount of [work](https://github.com/ipfs/notes/issues/37) put into a [tor transport](https://github.com/OpenBazaar/go-onion-transport/). Unfortunately, reliable anonymity requires more than just TOR:

* Even if it's identity changes, an IPFS node is easily fingerprintable through the content it stores.
* IPFS would need an "anonymous" mode that only enables features known to not leak information:
* No local discovery.
* No transports except TOR.
* Etc.
* A complete top-to-bottom security audit.
* Private routing (https://github.com/ipfs/notes/issues/291#issuecomment-396003860, https://github.com/ipfs/notes/issues/291#issuecomment-459876339) to make the network non-enumerable.

Unfortunately, this is a _lot_ of work and we have no plans on working on this till after the IPFS 1.0 release. Getting this wrong could put users in _serious_ danger so it's not something we'll even experiment with till we're ready.