Use concurrent builds

[vweevers]

As suggested by @lu4 in https://github.com/Level/rocksdb/pull/143#issuecomment-536828796:

What I've also found that significant performance improvements are obtainable from using concurrent builds i.e. node-gyp -j 16 rebuild build option. So I suggest to look closer into your node-gyp-build child and check whether it is possible to use the above option for build perf improvement.