Loader - parallel loading

Parallel loading is now supported, configured by
`loader.concurrentRequestCount`. Each file (a pack is now considered a
type of file) asset can be marked with `sync`, which is done for both pack
files and script files.

When a `sync` asset is encountered it must be loaded before any previous
resource any following resource is loaded (but it doesn't have to wait for
previous resources). Pack files are an exception in that they can download
(but are not processed) and they can fetch-around other `sync` assets.

Because of the concurrent nature there is no guarantee of the order in
which the individual events will file in relation to eachother, but local
ordering (e.g. onFileError always before onFileComplete) and overall
ordering (e.g. onLoadStart .. onFile? .. onLoadComplete) is preserved.

There is also increased error hardening and a few previous edge-cases
fixed (and likely a few bugs added).

Author: pnstickne