Add `source_connection_id` to the Process struct

[renatomassaro]

source_connection_id would work as an identifier of what connection originated the process (may be nil if the process is local).

Note that it's quite different from the already existing connection_id. connection_id points to the object of the process.

For instance, a player encrypting/hidding a connection, or starting an active firewall against a connection, would have that connection as the object, so connection_id would be used.

source_connection_id must be used in order to:

• enforce the process exists while the connection exists, so disconnecting from a process' source_connection_id would automatically kill it (though this behaviour may be overridden by the process).
• generate the logs with the proper trail, as defined by the bounce used on source_connection_id.

When implementing this, make sure to:

• Switch FileTransferProcess connection use from connection_id to source_connection_id, as the FTP connection created for the process acts as its origin connection. Same applies to BruteforceProcess, which creates a CRC connection.
• Add source_connection_id to FileInstallProcess.
• Review other processes
• Kill processes by default when source_connection_id is closed, but also give the option to override this behaviour (use internal SIGCONND or similar signal)
• Warn/ask users trying to log out a remote server with ongoing processes whether they would want to proceed ("Are you sure you want to log out? You have X processes that would be killed if you disconnect")
• Add this column on the DB as partial index (on NOT NULL). Review similar columns that could get optimized by this. Beware of cardinality.

[renatomassaro]

Minor change of plans:

I've decided to explicitly name process params as either src_* or tgt_*, where src represents stuff that originated the process, while tgt represents stuff that is targeted by the process. This will avoid dubious and potentially wrong code where process.file_id actually was meant to be process.target_file_id