Notifications: extract central Notifier, cordon off the internal Fanout implementation, and segregate instrumentation concerns

Revert "Create SyncListener. Since they do not rely on Thread, they can be used on Google App Engine."

Take a step back on this API direction.

This reverts commit 8104f65c.

Signed-off-by: NYehuda Katz <wycats@Yehuda-Katz.local>

Change Event#thread_id to #transaction_id. Defaults to one "transaction" per thread but you can explicitly declare the start of a new one. This makes it possible for each request to have it own id.

AS::Notifications.subscribe blocks are now yielded the arguments to pass to AS::Notifications::Event.new

This reverts commit 8cbf8254.