add DoneChan to the bootstrap config

[sanderpick]

As mentioned on the OB fork of go-ipfs: https://github.com/OpenBazaar/go-ipfs

"core/bootstrap.go add DoneChan to the bootstrap config which is closed when the inital bootstrap finishes. This is in place of blocking for the initial bootstrap."

So, it's an asyn operation instead of blocking. Most of the node's startup time is here, waiting for bootstrap, and it's quite variable. If we enable this too, the start up experience of the app will be much nicer:

• no need to wait for the node to come fully online to present the UI (thumbs, etc. are available offline)
• anything in SQLite can be available offline as well