move string_tokenize out of i2p_stream and add unit test for it

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9936 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix build

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9937 a83610d8-ad2a-0410-a6ab-fc0612d85776