[libcontacts] Revert "Cache the sorted index of contacts"

This reverts commit 29356f2857c12eb8c63878d04851c27588393846.
This is causing crashes/regression, so it isn't worth it as a performance
improvement.
[libcontacts] Ensure that previously retrieved details are maintained

Whenever we update a contact in the cache, copy any existing details
of the contact to the new copy, if those details are present in the
cache but not in the new query.
Remove obsolete variable

m_resultsRead is obsoleted by the fetch processed counts.
Merge pull request #41 from matthewvogt/detail-persistence

Detail persistence
[libcontacts] Allow empty avatar to precede others

The method for clearing an avatar is to store a local avatar with
an empty URL, which takes precedence over existing avatars.
Merge pull request #43 from matthewvogt/preferred-avatar

[libcontacts] Allow empty avatar to precede others