WL#3940: Print changed positions etc to error log

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium


1. Write new information and old position to error log so that users can
   see what hold before (and after) the change master command.

   James: "One fairly common error is to use the relay log number and
   position instead of the master binary log number and position when
   fixing a broken slave setup. Logging the master and relay log names
   and positions before and after CHANGE MASTER TO settings in the error
   log would be helpful when working out what has happened. The current
   log entries on shutdown and startup are already very helpful but not
   quite complete enough.

   For cases like some users, it would be good to have the slave write to the
   error log the master binary log name and position once an hour. Same
   for relay log name and position. We've seen cases where people seem to
   change the configuration files directly instead of using CHANGE MASTER
   TO. Sometimes by just copying the files from another slave."

2. Including the name of the slave server in the startup error log entry 
   would be helpful for some support cases.

3. Logging free disk space on the drive holding the binary logs on
   startup, shutdown and in hourly status entries in the error log would
   be helpful.

Related: BUG#43406