addr: added DnetAddr-from-string helper
backend: added backend control helpers: enable, disable, start defragmentation, set delay, make read-only and writable and read status
addr: we must use slice in DnetAddr, since len(addr) must correspond to the actual address length. But for map indexing we need a fixed-length structure, so introduce RawAddr with array instead of slice to hold address content
backend: implemeneted backend status reply parsing