[MRM-142, MRM-134] remove timestamp handling and instead simply index records that...

[MRM-142, MRM-134] remove timestamp handling and instead simply index records that aren't previously indexed. We can add timestamp checking for updating records later, but this removes the problems associated with relying on filesystem timestamps and a single tracking timestamp.
Included in this fix is some refactoring to reduce memory usage, including a hack to flush the project cache periodically. This hack can be removed when the project cache is configurable (see MRM-142).
This now indexes the ibiblio repository from scratch in ~23 minutes, and incremental indexing takes 30-60 seconds.

git-svn-id: https://svn.apache.org/repos/asf/maven/archiva/trunk@439935 13f79535-47bb-0310-9956-ffa450edef68