We had planned for a while to add an 'Overwrite' message which obtains
permissions without requiring retrieval of data. This is useful whenever
a master knows it will completely replace the contents of a cache block.
Instead of calling it Overwrite, we decided to split the Acquire type.
If you AcquirePerm, you MUST Release and ProbeAck with Data.
We used to place Monitors at the point of the ':='.
This was problematic because the clock domain might be wrong.
Thus, we needed to shove Monitors a lot.
Furthermore, now that we have cross-module ':=', you might not even
have access to the wires at the point where ':=' is invoked.
When we first implemented TL, we thought this was helpful, because
it made WidthWidgets stateless in all cases. However, it put too
much burden on all other masters and slaves, none of which benefitted
from this signal. Furthermore, even with addr_lo, WidthWidgets were
information lossy because when they widen, they have no information
about what to fill in the new high bits of addr_lo.
