fix: double pre start

[alanshaw]

This is a fix for preStart being called twice but also a partial refactor of the boot sequence.

The problem is that preStart always needs to be called between init and start and was actually being called in boot.js (via the IPFS constructor if repo exists or is newly initialized) as well as in start.js.

The following permutations of constructing, initializing and starting show how preStart was being called twice:

const ipfs = new IPFS({ init: false, start: false })
ipfs.on('ready', () => {
  await ipfs.init()
  await ipfs.start()
})

preStart is called ✅
preStart is called once from ipfs.start() ✅

const ipfs = new IPFS({ init: true, start: false })
ipfs.on('ready', () => {
  await ipfs.start()
})

preStart is called ✅
preStart is called once from boot.js (via the constructor) and again in ipfs.start() ❌

const ipfs = new IPFS({ init: true, start: true })
ipfs.on('ready', () => {})

preStart is called ✅
preStart is called once from boot.js (via the constructor) and again via boot.js since it calls ipfs.start() ❌

---

So having preStart called in boot.js and ipfs.start ensures it is always called, but in most cases it ends up being called twice.

This PR moves the call to preStart into init and ensures init is always called even if the repo already exists.

towards #1061
fixes #1162

[daviddias]

Code LGTM. I saw your question #1438 but if I'm seeing things correctly, it should not block this PR from being merged.

[alanshaw]

Correct :D