B-g#27501: 5.0 significantly more sys ("kernel") time than 4.1 \
      due to getrusage() calls

Even if profiling is turned off, the parser makes calls to reset 
the state at the beginning of each query.  That would eventually 
instantiate a PROFILE_ENTRY, which does indeed capture resource 
usage.

Instead, now check that profiling is active before progressing
far into the storage/expiration of old entries in the history.
This has the pleasant side-effect that queries to toggle profiling
are not recorded in the history.