Serialization of write operation in socket stream
NFD has problem similar to one in issue #1769:
async_send is not guaranteed to send all the supplied data and
async_write should be used instead. However,
async_write requires that no other async_write's are called until the first one finished. This implies that we need to serialize send() operations in at least StreamFace (datagram face may work as is).
The problem is not too critical, as it manifested itself so far only with UnixSockets, when transmitted packet was larger than 8192 bytes.
If solution is too complex, for 0.2 release we may just document the issue and really fix the problem in the next release.