[Pulp-list] Data model change: repository["packages"] to list of package ids only

[John Matthews]

I made changes to how we store packages under a repository document.  If there are no major objections I plan to check this in later today.

Pradeep and I have noticed a large performance issue when calling "_get_existing_repo()" from repo.py.  For rhel-i386-server-5 this takes roughly 30 seconds to fetch information on 7k packages resulting in a 10MB repository document being returned.  For Fedora 13 this is even larger and takes around ~90 seconds to fetch somewhere around 20k packages.

The issue is that we store a dictionary of "packages" under the repository.  The dictionary has a key of package id and a value of the full package object.  (Technically in mongo a reference to the package object is stored, not the full object.  When we fetch the repository through pymongo the AutoReference SON Manipulator fetches the contents of each package object).  This results in large repos being very expensive.  Further pulp relies on "_get_existing_repo()" in many places so this is a problem that will be seen often for large repos.

Over the weekend I made changes to how we store "packages", it's no longer storing packages as a dictionary, now we only store the package id in a list. 

"_get_existing_repo()" is much quicker as you can see:
 For rhel-i386-server-5 <only package ids>:
  Time:  .2 seconds  versus ~30 seconds
  Size:  1.5MB  versus 10MB

 For fedora 13 <only package ids>:
  Time: .3 seconds versus ~90 seconds
  Size: 2.5 MB versus 24MB

The result of fetching a repository object now will only yield "package ids" under "packages".  
If we want to flesh out all of the package objects as the call was previously doing, we can make a second call to the PackageAPI.  This is still much quicker than previous behavior.
 For rhel-i386-server-5 <full package objects>:
  Time: ~3 seconds versus ~30 seconds
  Size: 10MB and 10MB

 For fedora 13 <full package objects>:
  Time: ~7 seconds versus ~90 seconds
  Size 24MB and 24MB


Developers need to be aware repo["packages"] will only contains package ids.  It takes one extra call to flesh out the "packages" into their full objects, so if that's needed it's easy and not as expensive with the new approach.

I've made most of the changes needed for this, if there are no major objections I plan to check this in today.