add node cache, split dhtrunner.h from dht.h

CMakeLists.txt

include/opendht.h

include/opendht/callbacks.h

include/opendht/dht.h

include/opendht/dhtrunner.h

include/opendht/infohash.h

include/opendht/node.h

include/opendht/node_cache.h

include/opendht/securedht.h

include/opendht/utils.h

include/opendht/value.h

python/opendht.pyx

python/opendht_cpp.pxd

src/callbacks.cpp

src/dht.cpp

src/dhtrunner.cpp

src/node_cache.cpp

src/securedht.cpp

tools/dhtscanner.cpp

tools/tools_common.h