Bump the size of the squid cache_dir.

We were seeing disk space over limit errors on our CI
squid cache box:

 WARNING: Disk space over limit

So bumping the the cache_dir sizes to 4 Gigs...

Also switching to aufs for async IO.

Change-Id: I560decfe0d4770577141a43c4f16f7ead76ad2d4
Merge "Bump the size of the squid cache_dir."