Bug fixes

1) Fix memory leak when metric expires. The metadata metric is deleted
properly, but the value metric is not.
2) Memory leak because global_context is not destroyed before reloading
config.
3) Fix errornous handling of zip content. apr_socket_data_get()
returns APR_SUCCESS if GZIP_KEY is not present, but strm will 
be NULL and cause a seg-fault in line 275.