internal/router: bulk metadata API implemention
minor fixes for review
Merge branch '012-resolve-id-2' into 013-bulkmeta

Conflicts:
	internal/charmstore/store_test.go
internal/v4: add tests for bulk requests
Merge pull request #34 from rogpeppe/013-bulkmeta

internal/router: bulk metadata API implemention

We also fix all paths so that they start with / after splitting. This makes them compatible with the usual way that paths are specified in URLs.