Decouple tbuf and pickle.

Reorganize iproto parser (pickle) around two pointers at the request
parsing position and request end.
This makes the code more generic and easily usable in lua and
with new buffered I/O.

This also reduces the current widespread use of tbuf throughout
entire code base (good for future maintainability of the code).
Merge branch 'master' of github.com:mailru/tarantool