- Move data structures only needed by the default logger now (Action,
  Object) to logdefault.c

- Create petsclogdeprecated.h and petsc/private/logimpldeprecated.h
  for publicly visible symbols that are now deprecated.  Yes,
  lots of symbols in petsc/private/logimpl.h were not only visible
  but had docstrings.

- The macros PetscLogEventBegin(), PetscLogEventEnd(),
  PetscLogEventSync(), PetscLogObjectCreate(), and
  PetscLogObjectDestroy() now looks through the log handlers
  in PetscLogHandlers, sees if they have they appropriate
  callbacks (Exposed in PetscLogHandlerHot), and calls them.
  Up to 4 log handlers can run simultaneously.

- All queries about the registered things now go through
  petsc_log_state.

- Output functions dispatch to the appropriate PetscLogHandler

- There is no need for xmllogevent.c, xmlviewer.c, eventlog.c,
  classlog.c, and stagelog.c anymore.  Some orphaned functions
  (PetscLogSetThreshold(), PetscLogEventSetDof(),
  PetscLogEventSetError(), PetscASend(), PetscARecv(), PetscAReduce())

- PetscFEOpenCLLogResidual() did some intrusive things related
  to the old logging infrastruture: the private header
  for the default log handler is now imported into feopencl.c
  so that it can do equivalent things with the new infrastructure.

- PetscLogPauseCurrentEvent_Internal() is replaced with
  PetscLogEventsPause()

Deprecated symbols include:

- All functions related to PetscStageLog and the resizable array structs
  that it contained.

- The legacy EventBegin()/EventEnd() callbacks.

- PetscLogAllBegin(), which actually reported less information than
  PetscLogDefaultBegin() because it was not kept up-to-date with
  regards to GPU profiling.