Verilator path forward

[ktbarrett]

The in development version of Verilator (v5.025) introduces support for vpiInertialDelay, however it is up to the user to decide when to apply those writes. This means we are going to have to make changes to verilator.cpp. While I see those changes coming in the near future, cocotb will be broken with verilator until some fix is released.

If 1.9 is released soon, I suggest we introduce an ifdef for Verilator in out VPI implementation to use vpiNoDelay instead, which was the old behavior. If 1.9 is released soon there will only be a small gap where there is no support. This will also allow us to continue to support older version of Verilator that ignore the delay flag.

For 2.0 we will need to update the verilator.cpp.