Load manager action at transaction start

[chuck-h]

It would be possible for the load manager to compute available current, and adjust current limits if necessary, just as a new transaction commences. This could happen at authorization time. E.g. suppose

1. n vehicles are currently charging, and using the electrical feeder at full capacity
2. A new vehicle approaches and initiates a charging session; CP sends Authorize.req
3. CS evaluates tag id for validity, assume it passes
4. CS informs LM (load manager) that a new vehicle is about to connect
5. LM computes new reduced current limits for existing vehicle chargers, effectively "making space" for a new vehicle.
6. LM sends new current limits to CS
7. CS sends new current limits to CPs
8. CP sends StartTransaction.req
9. CS sends StartTransaction.conf
10. Charging of new vehicle begins.

[chuck-h]

This needs additional communication, beyond chuck-h/steve#4 to inform LM of new vehicle coming in.