Refactor NETCONF code after migrating to sysrepo2

Issue 2311 migrated the code to sysrepo2, but did a minimum amount of changes required to do so. Because of that, there are some parts of the code that are not completely adjusted to the new dependencies. Here are some of the proposed changes that would make the code more cohesive or that would just refactor the code for improved readability:

• The basic translators might not be able to translate all data types. The migration was done in a best-effort approach to apease the unit tests. But there is a noticeable lack of tests for unions and leafrefs for example. keatest-module has unions, but only one variant of it is tested. There is this discussion in the MR from 2311 that details one test case.
• Consider removing the LeafBaseType argument from TranslatorBasic::value(). The information available in the given ElementPtr might be enough to do the translation.
• Rename s_val to data_node where appropriate. The data type was changed from S_Value to DataNode, but the variable name was left the same to minimize changes. They are not equivalent. The former represents the value of a leaf or leaf-list member. The latter can represent an entire tree of data.
• Have references with meaningful names to service_pair.first and service_pair.second in src/bin/netconf. It's confusing what is one and what is the other. The first one is the module name, and the other is the kea-netconf configuration, I think.
• Rename SysrepoError to NetconfError. Errors can come from both libyang and sysrepo, so let's be inclusive.
• Consider changing ConstElementPtrs to ElementPtrs in src/lib/yang and src/bin/netconf and, with this, remove the need for const_pointer_cast. I like to squabble about the undefined behavior that these present, and if we're not removing them from everywhere, I would like to have it removed at least in the code that I am responsible for.
• Consider renaming methods in TranslatorBasic. I think their names can be more representative of what they do, especially after the migration.
• Solve doxygen warnings about functions with the same name in src/lib/yang.
• Use checkAndGetLeaf and checkAndSetLeaf where possible in translators.
• decode64 and encode64 are free-standing functions in translator.cc, but they can just as well be private methods inside TranslatorBasic.
• Consider doing something about NETCONF_SUBSCRIBE_NOTIFICATIONS_FAILED. It's an error message that shows up for the Kea modules because they don't have any notification nodes. kea-netconf continues regardless. We could at least turn it into a warning message.
• Solve the few TODOs in src/lib/yang.
• Replace EXPECT_NO_THROW with EXPECT_NO_THROW_LOG in src/lib/yang and src/bin/netconf for more insight when tests fail.
• Replace EXPECT_THROW with EXPECT_THROW_MSG in src/lib/yang and src/bin/netconf for stricter checking.