Speedup project graph build by paralleling operations

Motivation

In our project graph build time typically takes 14..15 seconds. 15-20 seconds penalty to run any operation is significant and could be improved. Currently it is a single threaded operation which could be improved. 

Proposal

• build graph concurrently with degree up to allowed by user with -T option

Preliminary results

• graph time improved from 14..15 to 5..6 seconds with 4 threads

Rejected options

• serialize graph state - require wide change to introduce serialization in maven domain model

Open design questions

• immutability in core classes to make concurrent implementation more safe