Change layer cache logic to target destination layers rather than sources.

In a future commit we'll fingerprint layers to detect changes.
Comparing modification times is not enough since some sources (for
instance Naturvårdsverket's SCI_Rikstackande) are updated on the server
even though no objects are being added; the source layer remains
unchanged but the file differs because of OBJECTID changes we are not
interested in.

Rather than using another cache layer/table for fingerprints, we cache
destination layernames rather than triplets (source_path, archive_member,
layername), along with the time at which the import was started rather
than source_path's mtime.

There is indeed no value in having exact source_path's mtime in the
cache.  What we need is simply a way to detect whether source paths have
been updated in a subsequent run.  Thanks to the shared locks the ctime
of any updated source path will be at least the time when the locks are
released, thereby exceeding the last_updated value.

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