Dirty Memory: Divide allow_stall into allow_delay and stall

[Yuval-Ariel]

in #549 , we've started by adding allow_delays == true to enable the new functionality of #423. That required extending the capabilities of the WBM to support all the combinations of allow_stall and allow_delays which was not prepared for the version so the #549 PR ended up with keeping the functionality in one flag.

Its best to add the new functionality of delaying based on memory usage under a new flag (allow_delays) and restore the previous allow_stall so that the behaviour is unchanged for users who want allow_stall which only does stalls.

in addition, the user expects the WBM to delay writes gradually and probably beyond the point of exceeding the memory capacity of the WBM. So the code needs to continue gradually decreasing the write rate even when the WBMs memory usage is beyond the capacity.