Host api
In this pull request, I add some routes for the host and the storage manager. These routes should be everything that is necessary for v0.6.0-rc1.
I also restructured modules/host.go a little, mostly to simplify things as much as possible. Overall, I'm quite happy with modules/host.go and modules/storagemanager.go (except for the expiryHeight thing), and this is just more polish on top of what was already pretty good.
Siac + testing to come shortly.