[SERVER-12174] Throttled mongoimport

This patch provides the functionality suggested in Jira issue SERVER-12174.

N.b. There are about a dozen lines of real change in src/mongo/tools/mongoimport_options.cpp, but I see in this github summary there is about a 100 lines change of whitespace reformatting. I guess this happened somehow during git rebasing and merging. Trying to apply the style guide 4-spaces-per-tab didn't eliminate it.