Optional ReadonlymaxUnrequested registers the planner may read to join two spans into one.
At the default of 0, a span holds only contiguous requested addresses.
Raising it trades reading registers nobody asked for against a lower
transaction count.
CAUTION: A gap can cover an address the device does not implement. Such a
span fails with ILLEGAL_DATA_ADDRESS and takes every address in it down,
including the ones that would have answered. Raise this only against a
register map that says the gap is readable.
Optional ReadonlymaxRegisters one read transaction may return. A span never grows past this.
Options for planReads.