Global tracker that will be used by the WALProcedureStore after load.
If the last WAL was closed cleanly we already have a full tracker ready to be used.
If the last WAL was truncated (e.g. master killed) the tracker will be empty
and the 'partial' flag will be set. In this case on WAL replay we are going
to rebuild the tracker.
If tracker for a log file is partial (see ProcedureStoreTracker.partial), we
re-build the list of procedures updated in that WAL because we need it for log cleaning
purpose. If all procedures updated in a WAL are found to be obsolete, it can be safely deleted.
However, we don't need deleted part of a WAL's tracker for this purpose, so we don't bother