respect file priority when selecting preview chunks

Previously, ChunkSelector would select preview chunks ahead of all
other chunks, regardless of file priority. This meant that files
prioritized as "Last" would nevertheless have all of their preview
chunks downloaded before all files prioritized as "First" or "Normal"
had finished downloading. This contradicts user expectation, which is
that scarce download bandwidth should not be devoted to files marked
"Last" until all files prioritized as "First" and "Normal" have been
completely downloaded.

This commit splits the existing PREVIEW_PRIORITY level into three
priority levels: FIRST_PREVIEW_PRIORITY, NORMAL_PREVIEW_PRIORITY, and
LAST_PREVIEW_PRIORITY. Chunks that ChunkManager determines to be
preview chunks have their priority levels boosted to the preview
priority level corresponding to their respective base priority level.

ChunkSelector's algorithm receives an overhaul that eliminates the
temporary std::list objects in favor of remembering the best (highest
priority, least number of downloaders) chunk during the iteration and
selecting that chunk if no chunk at the same priority level but with
no downloaders is found. The logic is ultimately equivalent to that
implemented in leastPeers(…) but will run faster since it requires no
heap allocations. Also, the new algorithm will not fail to select a
chunk that already has downloaders if there are no chunks that have no
downloaders. This should keep PieceDownloaders busier.