Permit "temp" plane category and clarify temp usages where most are actually "spill" type

Add "temp" to the schema for plane categories.
The vast majority of temp files in HPCC are actually spilt rows, either from inter-subgraph activity spills, or intra activity spills (e.g. sort phases), but there are a few cases where relatively small temp files are create that are not spills.

As it stands, it is not possible to specify a "temp" plane category and any getConfigurationDirectory(.. "temp" ..) will fallback to a "spill" plane if there is one.
Make engines use of "spill" usages explicit.