INCOMPATIBLE change: fetcher: rename fetcher_timestamp_enable setting back to...

INCOMPATIBLE change: fetcher: rename fetcher_timestamp_enable setting back to opt_fetcher_timestamp setting to fetcher_timestamp_enable and re-document its meaning. (It was not a good idea to mess with setting values in the constructor based on $_POST settings, which means that settings can be initialized differently in future HTTP requests.) Introduce getStartTimeStamp() instead to check whether the process uses a rolling timestamp. Also introduce setNextStartTimestamp() for abstraction of the actual storage, change feedIsComplete() with a second $ignore_timestamp parameter so it's more usable, introduce usesCachedItems(), and make checkCache() throw exceptions for strange cases.