Log plugin unloading at debug level

During server reconfiguration, plugin instances set up for the old views
are unloaded very close to the end of the whole process, after new
plugin instances are set up.  As the log message announcing plugin
unloading is emitted at the default "info" level, the user might be
misled into thinking that it is the new plugin instances that are being
unloaded for some reason, particularly because all other messages logged
at the "info" level around the same time inform about setting things up
rather than tearing them down.  Since no distinction is currently made
between destroying a view due to reconfiguration and due to a shutdown
in progress, there is no easy way to vary the contents of the log
message depending on circumstances.  Since this message is not a
particularly critical one, demote it to debug level to prevent
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merge request