Add hooks for EchoEvent's

BeforeEchoEventInsert can be used to abort or modify an
EchoEvent before it is created. If aborted, EchoEvent::create
will return false.
EchoEventInsertComplete can be used to run stuff once it's
been created.

Change-Id: I376708ac7aafc419bf87bceeff6de7de9bdc2f4e
Merge "Add hooks for EchoEvent's"