Only run processSnapsInBundle serially in domRunner
Only run processSnapsInBundle serially in domRunner

While my previous commit does make things render serially, it also
includes the bit where happo workers are processing as well. So
making this whole block run in serial will also cause happo workers
to run jobs in serial. Which will slow down jobs significantly.

Instead, we need to make this part run serially, and let the rest
happen in parallel.

enduire/happo.io,enduire/happo.io