Merge pull request #84 from rogpeppe/036-router-meta-put

internal/router: implement PUT for meta endpoints

This also adds the notion of a multi-error, adding a new field to the Error
type.
Fix intermittent test failures due to results ordering issues.

Make bundles-containing and charm-related API
endpoints return results in a specific order.
Use mongo query to sort entities in bundles-containing and charm-related.
Restore pointer in bundles-containing responses.
Merge pull request #85 from frankban/fix-intermittent-test-failures

Fix intermittent test failures due to results ordering issues.

Make bundles-containing and charm-related API
endpoints return results in a specific order.
internal/v4: implement extra-info GET and PUT