MINOR: compression/slz: add support for a pure flush of pending bytes

While HTTP makes no promises of sub-message delivery, haproxy tries to
make reasonable efforts to be friendly to applications relying on this,
particularly though the "option http-no-delay" statement. However, it
was reported that when slz compression is being used, a few bytes can
remain pending for more data to complete them in the SLZ queue when
built on a 64-bit little endian architecture. This is because aligning
blocks on byte boundary is costly (requires to switch to literals and
to send 4 bytes of block size), so incomplete bytes are left pending
there until they reach at least 32 bits. On other architecture, the
queue is only 8 bits long.

Robert Newson from Apache's CouchDB project explained that the heartbeat
used by CouchDB periodically delivers a single LF character, that it used
to work fine before the change enlarging the queue for 64-bit platforms,
but only forwards once every 3 LF after the change. This was definitely
caused by this incomplete byte sequence queuing. Zlib is not affected,
and the code shows that ->flush() is always called. In the case of SLZ,
the called function is rfc195x_flush_or_finish() and when its "finish"
argument is zero, no flush is performed because there was no such flush()
operation.

The previous patch implemented a flush() operation in SLZ, and this one
makes rfc195x_flush_or_finish() call it when finish==0. This way each
delivered data block will now provoke a flush of the queue as is done
with zlib.

This may very slightly degrade the compression ratio, but another change
is needed to condition this on "option http-no-delay" only in order to
be consistent with other parts of the code.

This patch (and the preceeding slz one) should be backported at least to
2.6, but any further change to depend on http-no-delay should not.

src/compression.c

@@ -350,6 +350,8 @@ static int rfc195x_flush_or_finish(struct comp_ctx *comp_ctx, struct buffer *out
 
 	if (finish)
 		b_add(out, slz_finish(strm, b_tail(out)));
+	else
+		b_add(out, slz_flush(strm, b_tail(out)));
 
 	out_len = b_data(out) - out_len;