WL#5819: Remove miscellaneous cross-references between replication and core

This is a step towards making replication a library, such that the 5.6 rpl-
lib can be plugged into the 5.5 core [WL#5675].

In this worklog, we move various functions that are defined in the wrong place, to 
the right place.

 - Move append_query_string away from log_event.cc, into core.

 - In sql_parse.cc, it creates an Incident_log_event. This is for debugging only
   and is used by two tests (rpl_incident.test and binlog_incident.test). This
   can be moved into rpl-lib.

 - net_request_file in rpl_slave.cc is used by sql_load.cc. It's not clear if
   this functionality is obsolete in rpl-lib or not. It should probably be moved
   to core, and either we remove it from rpl_slave.cc or we make core expose
   it so that rpl_slave.cc can use it.

 - In sql_load.cc we reference active_mi->rli->slave_patternload_file. This
   is only needed for an assertion. The assertion can just be removed.