Allow controling cache expiry timer

This change is aimed for libtorrent 0.16.10 release (and Windows platform mostly). 0.16.10 now properly returns expired cache entries to kernel, so memory used will now vary depending on the load instead of always growing to max cache as before. 'Auto' value for cache size can now be used w/o problems.

I'm not sure that 60s expiry is ok or too small: author states that default value is 300s, while the manual states it's 60s. (Anyway Deluge uses 60s by default)

Closes: #258