Implementation of the "extended interface" - to make use of the extended features of IPbus, E2bus or CBM DCA - how to implement RMW? #55

Implementation of the RMW functionality seems to be somehow difficult, if we want fully utilize caching.
The calling syntax on the bus interface level is:

rmw(self,address,mask,value,now=true) - schedules the read-modify-write
       operation defined as follows
       X:= (X and ~mask) | (value and mask)
       If "now" is true, the new value should be scheduled
       for writing.
       If "now" is false, the new value should be calculated
       and stored internally by the bus interface.
       The last call to rmw on the particular register should
       have "now" set to "true".

The idea is that we do not write the modified value immediately.
So a few operations may be concatenated, and only the final result may be written.
However the things are getting more complex if we realize that the read of the register value may return only the "future" that will be filled with real value only after dispatch.
It means, that the bus interface must accumulate the operations without knowing the initial value of the register...