MINOR: xprt_qstrm: implement Tx buffering

MINOR: xprt_qstrm: implement Tx buffering

This commit adds buffering on transmission for xprt_qstrm layer. This is
necessary in the rare case where send syscall only emits partial data.

A new <txbuf> member is defined in xprt_qstrm context. On first send
invokation, buffer is allocated and then the QMux transport parameters
frame is encoded. Then emission is performed via snd_buf and each time
the send function is invoked.