WL#3174: Bulk importer that uses multithreads for imports

Request from Brian Aker/Robin Schumacher for a new WL task:
Add support (or write a new) bulk importer that will use 
multithreads to handle imports.

Further details from an IRC conversation between Brian and 
Serg Golubchik, 2006-03-03:
[10:33]  serg: This is an idea me and Monty have been kicking around.
[10:33]  serg: Being able to bulk load dumps... put an index in the file
and use that to split the file up for threads.
[10:34]  and what does "multi-threading" in mysqlimport mean now ?
[10:35]  serg: All that is, is firing off X number of threads to do
multiple load data infile.
[10:35]  okay, so now it's one thread per file, but this WL means one file
in many threads ?