Get mongodb connector to new semver-major version

[dhmlau]

Due to the 2 breaking changes introduced in PR #462 and #417, here is what we are going to do:

• Get non-breaking changes PRs landed and release a new semver-minor / patch version (whichever applies), it includes:
  • support decimal128, support decimal128 #475
• Land the PRs with breaking changes and release a new semver-major version
  • Add case insensitive indexes support #417
  • Use new url parser by default #462
• Update available-connectors.json in loopback-workspace to use this new major version when adding a new datasource to LB applications
  • https://github.com/strongloop/loopback-workspace/blob/master/available-connectors.json

cc @jannyHou @b-admike

[jannyHou]

closing since all tasks are done.