fakesrc, identity, fakesink: do not generate last-message updates by default

fakesrc, identity, fakesink: do not generate last-message updates by default

Default to not creating lots of overhead by doing a couple of
g_strdup_printf()/g_free() per buffer or event just to generate
a last-message update that rarely anyone listens to. This means
that you need to enable silent=true explicitly in order to get
last-message dumps in gst-launch -v now. On the upside, people
won't inadvertently end up benchmarking g_strdup_printf()
performance instead of gstreamer data handling performance any
more.

Maybe the silent property should be renamed to enable-last-message
or something like that?