Formalize device address allocations and make better use of constraints

The device address constraints (or activation constraints) are used to assign a new device address to a device when it joins (or is personalized).

We currently use otaa, abp, world, local, private, testing, but nobody actually knows how to use this.

Proposal:

The `testing` range

The testing (00000000/03ffffff) range is reserved for development/testing.

All networks that allow test devices will announce this range
Test devices do not have to comply with the fair access policy
Traffic for test devices has the lowest quality of service
Not all gateways will allow downlink for test devices

The `private` range

The private (26000000-26000fff) is reserved for isolated private networks with their own private gateways. Traffic from this range will only be forwarded to the local private network components.

Private ranges are not announced, but are explicitly configured

The `local` range

The local (26xx0000/26xxffff) range will be forwarded to one public region (default)

Only one network will announce a range with the 26xx: eu=01, us-west=02, brazil=03, asia-se=04, ch=05, etc.

The `world` range

The world (26000000-2600ffff) range will be handled by all public regions (for "roaming" nodes)

All networks that allow "world" devices will announce this range
Allows devices to roam between regions without losing their session as they cross borders

TheThingsArchive / ttn