Add Mongo 3.x support and newer Node versions.

[kakato10]

• Add support for newer versions of mongodb. Update sqlite3 and level, so that model can work with newer node versions;
• Refactor some code related to the initialization of the connection to the db in the mongo and
  level adapters.
• The refactoring of level adapter was necessary due to the change of node_modules structure in the newer node versions.;
• All tests related to mongo, level and sqlite3 adapters ran successfully as well as the unit tests. Tested on Node 8.9.1 with Mongodb 3.6.3.