Support to mysql databases

[cphyc]

This brings back support to non-sqlite databases. Notably, this allows to run the database on one server with the data on another.

TODO:

• [x] rename environment variables to TANGOS_

[cphyc]

As of 277a44790ac8ddbc2c283ad350dc1d44d472ae7d with sqlalchemy 1.3., here is a non-exhaustive list of erroring tests

• sqlite:
  • test_ahf_trees.test_ahf_merger_tree_import
  • test_consistent_trees.test_consistent_tree_import
  • test_linking.test_crosslinking
  • test_hop_strategy.test_multisource_with_nones_as_temptable
  • test_live_calculation.test_match
  • test_live_calculation.test_calculate_array
  • test_live_calculation.test_liveproperty_requiring_redirection
  • test_live_calculation.test_non_existent_redirection_multihalo
  • test_live_calculation_link_syntax.test_link_returned_halo_is_valid
  • test_live_calculation_link_syntax.test_link_missing_data
  • test_live_calculation_link_syntax.test_link_can_be_used_within_calculation
• MySQL:
  • test_db_writer.test_parallel_writing
  • test_hop_strategy.test_multisource_with_nones_as_temptable
  • test_stat_files.test_insert_properties

[apontzen]

I fixed a performance issue and have left two ultra-minor comments above - as soon as they are done I think this is good to merge. Hooray!