refactor(connections): first step in refactoring connections to a more generic/understandable codebase
refactor(connections): fix a few weird bugs and tests
refactor(connections): fix maria connections, fix more tests
refactor(connections): sqlite needs to be singleton for regular queries but needs a differerent connection per transaction
refactor(connections): remove obsolete files
refactor(connections): add test for handling multiple parrallel transactions in a row
refactor(connections): changelog entry
refactor(connections): fix sqlite3 missing package error
Merge pull request #1892 from sequelize/connection-management-refactor

Refactor connection handling - Closes #1601