cel: add local optimization begin event

The current AST_CEL_LOCAL_OPTIMIZE event is and has been
triggered on a local optimization end to serve as a flag
indicating the event occurred.  This change adds a second
AST_CEL_LOCAL_OPTIMIZE_BEGIN event for further detail.

Resolves: #52

UpgradeNote: The existing AST_CEL_LOCAL_OPTIMIZE can continue
to be used as-is and the AST_CEL_LOCAL_OPTIMIZE_BEGIN event
can be ignored if desired.

UserNote: The new AST_CEL_LOCAL_OPTIMIZE_BEGIN can be used
by itself or in conert with the existing
AST_CEL_LOCAL_OPTIMIZE to book-end local channel optimizaion.

(cherry picked from commit 8d3d8fcdcd79401dc14c497c251e565fa4481e41)