Support multi cache reads and writes

[julianrubisch]

I.e., implement read_multi_entries, write_multi_entries, and delete_multi_entries

@oldmoe I'd be happy to help where I can

[oldmoe]

It should be pretty straightforward, I will make that happen while adding proper tests to litecache

[julianrubisch]

I think this would also especially benefit from a benchmark, since the gains are usually quite impressive compared to "normal" reads/writes