feat: add StringWithConcurrency function

[mdross95]

Proposal for adding StringWithConcurrency function to file_instance.

When testing metro2 file generation with a large amount of segments with moov metro2, I noticed that the String() function in file_instance can take a decent amount of time (i.e. ~15 minutes on my machine for close to ~200k segments).

To alleviate this, I implemented a simple StringWithConcurrency function in our fork that takes in a desired # of concurrent goroutines, splits up the base segment processing into that amount of chunks, processes them in parallel, and merges them at the end.

In my testing, leveraging this function has allowed us to reduce the String() generation time by over tenfold.

We are now using this function in our fork for furnishment process - I figured I would open a PR if moov thinks it is valuable to the main repo. Please let me know what you think - thanks!

[adamdecaf]

This makes sense. Thanks!

[adamdecaf]

I'm thinking that ConcurrentString(isNewline, goroutines) sounds better. Thoughts?

[adamdecaf]

Also, is there a reason we don't default this as runtime.NumCPU() ?

[mdross95]

ConcurrentString works for me, and sure, we could default it to that in String() instead of sending 1 👍

[adamdecaf]

eb9fc66