DAS cache: more checks, less confusion

Fixes #79 (closed)

• automatic validity check: if the db value changes, the query will be redone and the cache file updated
• --redodbqueries and --overwritesamplefilelists removed: to expire the cache can be deleted or removed; if a cache is configured and the queries are done, they will fill it (this is hopefully sensible behaviour, and more predictable than having the options)
• top-level dbcache feature: if specified, it will be used to cache the results of all queries for samples with db (in <dbcache>/<sample>txt, so also renaming samples will expire them); if files is still specified as well, a warning will be printed (it will be ignored).

Anything I missed? More testing is also welcome (this should be independent of all other open PRs, but can stay open for a while to allow for testing)