Add multithreaded support to mongoimport

There are a couple possible approaches to take here:

1. add a user configurable knob (pros: easy and easily tunable; cons: easy to misuse)
2. do an adaptive solution (pros: just works; cons: tricky to get right)

Other considerations:

1. It will be easy to overload the server
2. Need to change how users throttle writes
3. If it is a requirement that the import is sequential according to the source file, parallel import is not possible

Because mongoimport works against a single collection, it is the easiest tool to parallelize. Simply read-ahead from the source into memory and use >1 connection to the server.