Make initiator clear in connection_started

[rmarx]

Currently, connection_started utilizes src_ip and dst_ip (and equivalent for ports), where the intended meaning is that src means the initiator (typically client) and dst the server.

However, this is a) unclear and b) it does not allow deducing what the "local" ip is for where this trace was captured without correlating client/server with the vantagepoint (and if that vantagepoint is network, it becomes more murky).

Some options for solving:

1. rename fields to local_ip and remote_ip
2. add initiator field (e.g., with values 'local' and 'peer')
3. completely revise how we log paths/IPs and potentially decouple from connection_started (also good in terms of privacy, migration, multipath, ...).

Option 3 is probably best, should revisit when properly fixing migration in qlog (see also #79 and #57)

cc @marten-seemann