(#2553) - parallelize bulkDocs for idb/websql

Here it is. I'm definitely not expecting this to make it into 3.0.0, because I want you guys to go over it with a fine-toothed comb. It's a pretty big increase in code complexity for modest gains (5-25%), so I really want to split hairs and make sure my variable names make sense and stuff.
Nope, we just bail out after the first batch.