Adds a benchmark that exercises the router's partitioning DmlHandler
implementation against a set of three files (very small, small, medium)
with 4 different partitioning schemes:
* Single tag, which occurs in all rows
* Single tag, which does not occur in any row
* Default strftime formatter (YYYY-MM-DD)
* Long and complicated strftime formatter
This covers the entire partitioning overhead - building the formatters,
evaluating each row, grouping the values into per-partition buckets, and
returning to the caller, where it normally would be passed to the next
handler in the pipeline.
Note that only one template part is evaluated in each case - this
measures the overhead of each type of formatter. In reality, we'd expect
partitioning with custom schemes to utilise more than one part,
increasing the cost of partitioning proportionally. This is a
lower-bound measurement!
a873e119c4