eitd: remove unused FreesatTables.hpp
eitd: Reduce sectionsd memory requirements:

* Various EPG related strings (language- and country keys, audio descriptions) come with a pretty low variance. Cache them, and use an index instead of allocating redundant strings for each and every SIevent.
* Storing content classifications in native format instead of std::string removes quite some memory overhead, too.

This saves about 250 bytes of RAM for every event cached, resulting in a considerably reduced sectionsd memory footprint.
Merge remote-tracking branch 'martiis-neutrino-mp/master'