lib: fix g_hash_table related read-after-free bug

lib: fix g_hash_table related read-after-free bug

The two g_hash_table functions (insert, add) have different behaviour
with respect to existing keys. g_hash_table_insert frees the new key,
while g_hash_table_add (which is really g_hash_table_replace in
disguise) frees the existing key. With this change 'ref' is live until
the end of the function (assuming single-threaded access to
'hash'). We can't guarantee it will continue to be live in the
future (i.e. there may be a future key duplication) so we copy it with
the allocation context passed to parse_references (in practice this is
the notmuch_message_t object whose parents we are finding).

Thanks to Tomi for the simpler approach to the problem based on
reading the fine glib manual.