merge graceful pause fix from aio
merged piece request fix from RC_0_16
merge header fix from libtorrent_aio
remove resolve countries build configuration from the public API (make linking easier)
remove disable-dht build configuration from the public API (simplify linking)
disable pool allocators does not affect libtorrent API
move out alert_manager to its own compilation unit. remove TORRENT_DISABLE_EXTENSIONS from affecting the public API