Load manifests asynchronously (#3894)

* Refactor manifest loading to use new async Process.popen()

* Use DelayableAction to ensure deferred actions are managed correctly

DelayableAction represents an action on a target that can be deferred
until a more appropriate time. In this case, we use defer to cleanup
objects in the case of early return error handling, but we need to
delay this action when passing the target for use into an async
completion block.

* Use delegateQueue to avoid reusing the TSCBasic.Process default queue

The manifest parser uses SourceControl.GitShellHelper synchronously,
which would cause a hang if using the default queue for completions.

Author: pcbeard