Add the ability to build a cross-platform rocksdbjni-dev

[lolski]

We need to add the ability to build a cross-platform rocksdbjni-dev (Windows, Linux, OS X).

Given that it can only be achieved by building the library on each platform (eg., you can only get a Windows library by performing the build step on a Windows machine), we should approach the problem by automating the whole build steps in the Grabl CI:

• Fork https://github.com/facebook/rocksdb and update the Makefile so that it doesn't strip the debug flag (NOTE: we should submit a PR to RocksDB so that we can eventually remove the fork)
• Move //library/rocksdbjni-dev onto its own repository, rocksdbjni-dev-builder
  • Have a Grabl release pipeline that when approved, deploys a cross-platform JAR - the pipeline should spawn four CI jobs: deploy-windows, deploy-mac, deploy-linux, and deploy-all.
  • deploy-mac must ran on a Mac machine. Given that it's not possible to do so in Azure, we'll have to spawn the job in CircleCI
  • deploy-all should download the deployed library from the other three jobs and combine them into a single final JAR. This final JAR is what should be included in Grakn Core distribution.