BUG/MINOR: stream-int: make sure to always release empty buffers after sending

BUG/MINOR: stream-int: make sure to always release empty buffers after sending

There are some situations, after sending a request or response, upon I/O
completion, or applet execution, where we end up with an empty buffer that
was not released. This results in excessive memory usage (back to 1.5) and
a lower CPU cache efficiency since buffers are not recycled as fast. This
has changed since the places where we send have changed with the new
layering, but not all cases susceptible of leaving an empty buffer were
properly spotted. Doing so reduces the memory pressure on buffers by about
2/3 in high traffic tests.

This should be backported to 2.0 and maybe 1.9.