chore(insight): Fully qualify class names with `use` or leading backslash

In general, missing `use` statements can lead to bugs or at least ambiguity.
Better to fully qualify all classes.

See "Missing use statement should be avoided" on this report:
https://insight.sensiolabs.com/projects/0630ff8f-809b-4409-a718-12f7da91367e/analyses/5
Merge pull request #7477 from ewinslow/missing-use

chore(insight): Fully qualify class names with `use` or leading backslash
chore(config): Move `$CONFIG` to member variable for most classes
Merge pull request #7479 from ewinslow/eliminate-globals

chore(config): Move `$CONFIG` to member variable for most classes
chore(cache): Add some nice new internal APIs for caching