Simplify OperationsContext with yangtools 7.0.9+

Simplify OperationsContext with yangtools 7.0.9+

Simplified the logic in OperationsContent class with usage of
features available in yangtools 7.0.9.

Now we use EffectiveModelContext#findModuleStatements to find module
statements already sorted by revision.

JIRA: NETCONF-927
Change-Id: I5f0d3e310ac6cf499cb93c6e47e7142cf8653962
Signed-off-by: OleksandrZharov <Oleksandr.Zharov@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 45a89bde4d8159468a6a1211b78cb421aab5fbc4)