codeintel: Replace encoding/json with json-iterator/go for reading LSIF

Created by: efritz

Replace encoding/json with json-iterator/go for unmarshalling LSIF encoded as JSON lines. This speeds up the conversion considerably without any engineering effort.

Running the precise-code-intel integration suite's upload phase on master:

3m37.597s
3m35.858s
3m42.591s

And after this change:

2m59.991s
2m54.612s
2m56.651s