Add EOS log messages for PREPARE workflow events

See operations issue #513.

EOS MGM handles PREPARE workflow events: staging xrdfs -s, abort xrdfs -a and eviction xrdfs -e

Currently there are no log messages in EOS when these events are processed/passed to CTA. We should add them.

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Corresponding EOS issue: https://its.cern.ch/jira/browse/EOS-5360