gossip: gossip_load: Use ets directly, not via PDB

Use ets directly instead of via PDB, because gossip_load uses multiple ets tables.
gossip: send back discarded messages

The callback module can instruct the behaviour module to send back discarded
messages via the send_error mechanism to prevent mass loss at the sender.
gossip: Fix web_debug_info in gossip2

gossip_load was hardcoded in on_active({web_debug_info,...}), now collects info
from all callback modules.