repo: refactored repository interfaces (#780)

- `repo.Repository` is now read-only and only has methods that can be supported over kopia server
- `repo.RepositoryWriter` has read-write methods that can be supported over kopia server
- `repo.DirectRepository` is read-only and contains all methods of `repo.Repository` plus some low-level methods for data inspection
- `repo.DirectRepositoryWriter` contains write methods for `repo.DirectRepository`

- `repo.Reader` removed and merged with `repo.Repository`
- `repo.Writer` became `repo.RepositoryWriter`
- `*repo.DirectRepository` struct became `repo.DirectRepository`
  interface

Getting `{Direct}RepositoryWriter` requires using `NewWriter()` or `NewDirectWriter()` on a read-only repository and multiple simultaneous writers are supported at the same time, each writing to their own indexes and pack blobs.

`repo.Open` returns `repo.Repository` (which is also `repo.RepositoryWriter`).

* content: removed implicit flush on content manager close
* repo: added tests for WriteSession() and implicit flush behavior
* invalidate manifest manager after write session

* cli: disable maintenance in 'kopia server start'
  Server will close the repository before completing.

* repo: unconditionally close RepositoryWriter in {Direct,}WriteSession
* repo: added panic in case somebody tries to create RepositoryWriter after closing repository
  - used atomic to manage SharedManager.closed

* removed stale example
* linter: fixed spurious failures

Co-authored-by: Julio López <julio+gh@kasten.io>

cli/command_diff.go

@@ -21,7 +21,7 @@
 	diffCommandCommand   = diffCommand.Flag("diff-command", "Displays differences between two repository objects (files or directories)").Default(defaultDiffCommand()).Envar("KOPIA_DIFF").String()
 )
 
-func runDiffCommand(ctx context.Context, rep repo.Reader) error {
+func runDiffCommand(ctx context.Context, rep repo.Repository) error {
 	ent1, err := snapshotfs.FilesystemEntryFromIDWithPath(ctx, rep, *diffFirstObjectPath, false)
 	if err != nil {
 		return errors.Wrapf(err, "error getting filesystem entry for %v", *diffFirstObjectPath)