Add a new notification on_before_allocate_tablet_map that is called when
creating a tablet map for a new table and passes the tablet map.
This will be useful next for CDC for example. when creating tablets for
a new table we want to create CDC streams for each tablet in the same
operation, and we need to have the tablet map with the tablet count and
tokens for each tablet, because the CDC streams are based on that.
We need to change slightly the tablet allocation code for this to work
with colocated tables, because previously when we created the tablet map
of a colocated table we didn't have a reference to the base tablet map,
but now we do need it so we can pass it to the notification.
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