Migrate to a numeric model::ntp

[dotnwat]

This might be achievable in a two step process where the first step would be to store a database of model::ntp -> model::old_ntp on each core and then in the next step to eliminate the string based version entirely accept for printing or dealing with file systems and clients.

Other ideas here on a good migration process?

What would the implications be in raft and controller? @mmaslankaprv

[emaxerrno]

@dotnwat the other alternative would be for ntp to be an id into a string name. a sharded<ntp_reference> type