Parallelize fetch and expand phase across APKs

[jonjohnsonjr]

Building on #773

We currently do something that looks like this:

def installPackage(pkg):
  apk = fetchPackage(pkg)
  files = ExpandApk(apk)
  installFiles(files)

def installPackages(packages):
  for pkg in packages:
    installPackage(apk)

Where we do fetch, expand, and install serially for every package.

The only part that really needs to happen serially is the installFiles step. We can concurrently fetch and expand all the packages, then serially run installFiles on all the results.

We probably want the fetch and expand stage to return a promise-like thing so we can start on the installFiles stage as soon as the next result has finished being fetched and expanded.

Here's some kind of weird go python js mashup of what I mean:

def async fetchAndExpand(pkg):
  apk = fetchPackage(pkg)
  files = ExpandApk(apk)
  yield files

def installPackages(packages):
  todos = make([]todo, 0, len(packages))
  for i, pkg in packages:
    go func():
      todo[i] = fetchAndExpand(pkg)

  for todo in todos:
    files = await todo
    installFiles(files)