[fuzz] txdownloadman and txdownload_impl

The txdownload_impl is similar but allows us to check specific
invariants within its implementation. It will also change a lot more
than the external interface (txdownloadman) will, so we will add more to
this target later.

src/test/fuzz/CMakeLists.txt

@@ -117,6 +117,7 @@ add_executable(fuzz
   timeoffsets.cpp
   torcontrol.cpp
   transaction.cpp
+  txdownloadman.cpp
   tx_in.cpp
   tx_out.cpp
   tx_pool.cpp