This includes two features that can help one actually complete a cosmological simulation with conduction enabled.
Dynamic Hierarchy Rebuilding: this causes the hierarchy on a given level to be rebuilt on a timescale that is calculated ignoring the conduction time. The conduction time is still used to determine the actual timestep to be used, but the hierarchy will not be rebuilt until the above time period has passed. This is reasonably valid since conduction should not considerable change the baryon fields which control the need to refine. This results in a decent speedup and also cuts down on memory fragmentation. This machinery could also be extended to include other machinery that causes short timesteps but does not necessarily change the density fields too much.
Light crossing time limiter for conduction. This sets a floor for the conduction timestep of the light crossing time (dx / c). This eliminates extremely small conduction timesteps from hot, low density gas and does not significantly change the results.
Update 1: fixed @samskillman's catch on the mixed return values in Grid_ComputeConductionTimestep.
Is there any reason this cannot be merged?
I still have several comments below I'm waiting for a response on.