Server send events are send as chunked responses. The expectation is
that the chunks are immediately send and not buffered until the whole
response is done.

If a response is detected, the chunks are normally read and each read
chunk is immediately flushed, replicating the upstream behavior
downstream.

Comments.

Supporting changes:
- add jetty as a test dependency used as servlet execution engine
- bump javax.servlet-api to version 3.1.0 which is required by jetty

Jetty 9.4 requires java 8 as minimum version.