Modify current realization to support concurrent processing

[batovpasha]

Need to modify the current realization in that way it should process input string concurrently by chunks using threads(worker_threads).

The following requirements should be met:

• Function must accept optional options object as a second parameter with threads option that has default value 2(number of threads that supports by our AWS lambdas)
• Function must process a string in a single thread if the string is enough small(need to find that size, approximately it is ~33kb)

[batovpasha]

A good tool for the implementation: https://github.com/wilk/microjob.