Add the file name list to the unified message metadata pass.

Even if the caller never uses the file names, there is little cost to
simply fetching the file name terms.  However, retrieving the full
paths requires additional database work, so the expansion from terms
to full paths is performed lazily.

This also simplifies clearing the filename cache, since that's now
handled by the generic metadata cache code.

This further reduces my inbox search from 3.102 seconds before the
unified metadata pass to 2.206 seconds (1.4X faster).

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