Add agent support to import benchmark.

[jaffee]

The import benchmark will be much more useful if we can support multiple agents importing different data simultaneously. Here are some ideas for things agent number could control:

• the set of columns being imported into
• the set of rows being imported into
• the frame being imported into
• the index being imported into

Implementation would look something like:

1. Add a --agent-controls flag to cmd/import.go which is a string with four options (columns, rows, frame, index)
2. in bench/import.go modify the Init method to set up the benchmark's configuration based on the agent controls flag.
   • probably have a switch statement with four cases (and a default that returns error)
   • columns would change the base column id and the max column id to be offset by number of columns times agent number. Similar for rows.
   • frame and index would create a new frame or index by concatenating the frame or index name with the agent number