Speedups & memory improvements

I like cscope, but had migrated to exctags for Erlang.  I reworked some of your code to process each file in it's own process to take advantage of multi-core.  Because processing many files in parallel takes more memory, I worked on that a bit too.

In short, it took >6 minutes and 85MB for me to build a cscope DB of all OTP sources and my project (2503 files).  I got it down to 18 seconds and 133MB.