New Iterator API

doc/user/iterator-types.xml

doc/user/stored-procedures.xml

src/box/box.lua

src/box/box.m

src/box/box_lua.m

src/box/index.h

src/box/index.m

src/box/request.m

src/box/tree.m

src/memcached.m

test/big/iterator.lua

test/big/iterator.result

test/big/iterator.test

test/big/lua.result

test/big/lua.test

test/big/tarantool.cfg