DatastoreAccess: add support for generic user-defined RPCs

I was lazy and in the NETCONF backend, the fillMap patch does not check
for actual string prefix match, it just optimistically trims the RPC
prefix. I think this is safe (definitely in the "won't crash" department
due to use of std::string::substr(), but also in the "won't produce
garbage" context because libyang is expected to validate everything).

In the sysrepo backend, I had to introduce some duplication into that
visitor which converts from our data types to sysrepo. It tirns out that
sysrepo::Session::set_item requires a separate S_Val, whereas in context
of handling an RPC's output, we have a Vals_Holder which, after
reallocation, becomes a Vals instance, and that one does not support
replacing the individual Val instances by "something" -- one has to call
a Val::set, and these methods are different from Val::Val constructors.

I'm explicitly testing for lists and containers because the
documentation looked a bit scary -- I understood it in a way which make
me spend extra effort to make sure that all that has to be created gets
created.

Change-Id: I717af71d69b209c444e1c5fe6d8ec2c2fcbdde8b
diff --git a/src/netconf_access.hpp b/src/netconf_access.hpp
index a5e0485..f43976a 100644
--- a/src/netconf_access.hpp
+++ b/src/netconf_access.hpp
@@ -41,6 +41,7 @@
     void deleteListInstance(const std::string& path) override;
     void commitChanges() override;
     void discardChanges() override;
+    Tree executeRpc(const std::string& path, const Tree& input) override;
 
     std::shared_ptr<Schema> schema() override;