Register only one events servlet

Register only one events servlet

SubscribeToStreamUtil and the corresponding servlet are strongly
related -- the former produces references to the latter.

Express this dependency by allocating both object in the same place and
pass them to their respective users.

This has the nice end result that we do not enable WS servlet when
configured to use SSE and vice-versa -- making it clear the two belong
together and paving the way towards making the two proper components.

Change-Id: I42b0878fa9c9ac4e4b5800bfde8719134a409c7a
JIRA: NETCONF-1102
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>