Cleanup SHAMap and simplify interfaces:

* Improve error reporting (more readable exception messages)
* Reduce function complexity (split oversized function to smaller pieces)
* Reduce code duplication
* Reduce buffer copying

src/test/shamap/SHAMapSync_test.cpp

@@ -140,7 +140,6 @@ public:
                              .addRootNode(
                                  source.getHash(),
                                  makeSlice(*gotNodes_a.begin()),
-                                 snfWIRE,
                                  nullptr)
                              .isGood());
         }