ACU: record scan parameters in some feed

[mhasself]

As the agents starts a scan, it should record the key scan parameters (vel, accel, drift; anything pertinent for more complex scans) to some low rate feed. That info can then more easily be put into, e.g. the obs book metadata. This is helpful because:

• downstream users may want to collect obs with certain scan parameters, easily.
• depending on ACU configuration, the velocity or accel may be lower than what was actually requested, and this records the necessary info to check that.