Changes since previous 1.2.x release.
- fix issue where stop-when-ready would not close files
- uTP performance, fix packet loss when sending is stalled
- uTP performance, prevent premature timeouts/resends
- uTP performance, more lenient nagle's algorithm to always allow one outstanding undersized packet
- uTP performance, piggy-back held back undersized packet with ACKs
- uTP performance, don't send redundant deferred ACKs
- fix wanted_done/done accounting when force-rechecking
- expose userdata via torrent_handle (back-port from 2.0)
- fix renaming of filenames that are too long for the filesystem
- made UPnP and LSD code avoid using select_reactor (to work around an issue on windows in boost.asio < 1.80)