`gossip_store_add` is the entry point for messages from the network, so it
should do the bookkeeping and disable on failures. `gossip_store_append` is the
shared function that wraps messages and writes it to the given file. This is
shared between the from network path and the compaction path, so we don't
directly use the `gossip_store` instance, but `fd`s.
Signed-off-by: Christian Decker <decker.christian@gmail.com>