Refactored archive data querying code. Removed tree-like data structure from...

Refactored archive data querying code. Removed tree-like data structure from Piwik_Archive, added memory optimizations for archiving (not storing archive processing instances), added optimization for querying data indexed by site + date, added ability to select multiple blobs/metrics at once (from different plugins), added optimization to Piwik_ArchiveProcessing_Period where numerics/records are selected all at once instead of one at a time.

Also added data access layer object for archive querying code, allowed anonymous function objects to be used as DataTable filters, and removed 'timestamp' DataTable metadata.