ServerSentEventHttpMessageReader had logic to split on new lines
and buffer until an empty new line (start of a new event). To account
for random data chunking, it later re-assembled the lines for each
event and split again on new lines. However bufferUntil was still
unreliable a chunk may contain nothing but a newline, which doesn't
necessarily mean an empty newline in the overall SSE stream.
This commit simplifies the above by delegating the splitting of the
stream along newlines to StringDecoder.
Issue: SPR-16744
da98ff72d2