Parse the config file on arenas

read_jconf now reads the file and builds the JSON tree in a scratch arena
that is released before it returns, and copies the parsed strings into one
process-lifetime arena instead of a malloc per string. parse_addr and the
config parser share one address splitter.

Port test_jconf to C++ and cover every field copied out of the parse tree,
a re-read, and the densest file the size limit accepts.

scripts/no_heap_baseline.txt

@@ -3,7 +3,7 @@
 # violations, and also when it loses some, so the number here always goes
 # down. Remove a file once it is rewritten on the src/core primitives.
 src/ipset.cpp 3
-src/jconf.cpp 16
+src/jconf.cpp 6
 src/json.cpp 2
 src/local.cpp 1
 src/manager.cpp 14