Move NetconfServerDispatcher to netconf.server.api
There is no point in polluting netconf-api with server-side constructs,
this move takes the first step in sanitizing the dependency tree.
Unfortunately we cannot move NetconfServerDispatcherImpl, as it is tied
down by mdsal-netconf-impl's configuration compatibility constraints
(i.e. the configuration is tied to NetconfMonitoringServiceImpl).
JIRA: NETCONF-945
Change-Id: I0d1b3631d2af50ca7291bfca06a9ddd490c26a71
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>