LOG_GC_TIMESTAMPS: Reduce default verbosity.

Hide "GC cyclic reference in %O." behind LOG_GC_VERBOSE.
Merge branch 'grubba/gc-logging' into devel

* grubba/gc-logging:
  LOG_GC_TIMESTAMPS: Reduce default verbosity.
LOG_GC_TIMESTAMPS: The histogram is no longer accumulative.

The local gc behaviour is usually more interesting than the global.
Merge branch 'grubba/gc-logging' into devel

* grubba/gc-logging:
  LOG_GC_TIMESTAMPS: The histogram is no longer accumulative.