Remove voodoo code in _raw_metadata

This doesn't avoid a memory leak at all, and removing it doesn't break any
tests.
No need to explicitly allocate a decode_all struct since MMDB_get_tree already does this

This seems to fix the memory leak, which makes sense. We were allocating the
decode_all_s struct twice, once in the XS code and once in libmaxminddb, but
we only freed it once.
Add a README.dev file with some info about valgrind and a suppressions file for valgrind