Eliminate RestconfSchemaServiceImpl

Eliminate RestconfSchemaServiceImpl

RestconfSchemaServiceImpl is serving YANG/YIN schema sources. While this
capability is not standardized, integrate it into RestconfServer as
modules{Yin,Yang}GET().

This turns out to be a major refactor, as the implementation was shoddy
-- it required local DOMSchemaService to expose
DOMYangTextSourceProvider extension and then it used that when talking
to mount points.

This refactor integrates the ability to lookup sources into
DatabindContext, with the corresponding DOMDatabindProvider wiring. If
no DOMYangTextSourceProvider is present, we call back to using
YangTextSnippet.

JIRA: NETCONF-773
Change-Id: I57689b2915eda0486c0bb5d5ba6de992b3107c0b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>