Improve Thumbnail Performance

[cxallan]

The thumbnail performance in milestone:3.0-Beta3.0 is quite degraded; partially due to #1047 but mainly because of non-intelligent database querying.

[cxallan]

#2662 implements the following improvements:

• Shallow retrieval throughout to reduce database overhead for cached thumbnails
• Bulk retrieval for bulk thumbnail methods (getThumbnailSet() and getThumbnailByLongestSideSet())
• Deferred renderer creation
• Cleanups to data retrieval hierarchy
• Moved certain log messages into DEBUG
• Cached enumeration retrieval (single set of enumeration retrievals per thumbnail bean instance)

[cxallan]

Sorry r2662.

[jmoore]

See r2678 for a possible solution to the flushing issue in getThumbnailSet. If iupdate.commit() interferes with other logic in the other methods, then probably the only solution is to use SessionFactory directly and manage the session yourself (barring the Executor).

[cxallan]

r2789 should now make this about as fast as we're going to get it without a lot more work.

[cxallan]

All the multi-100ms problems have now been resolved. Difficult tasks still remain that could increase performance:

• Server-side in memory caching
• Thumbnail state cache that is database ignorant
• Various I/O pooling techniques
• etc.

Closing until a time at which it's deemed we need to take this to the next level.