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_cache_sync.go

@@ -8,10 +8,10 @@
 
 var cacheSyncCommand = cacheCommands.Command("sync", "Synchronizes the metadata cache with blobs in storage")
 
-func runCacheSyncCommand(ctx context.Context, rep *repo.DirectRepository) error {
-	return rep.Content.SyncMetadataCache(ctx)
+func runCacheSyncCommand(ctx context.Context, rep repo.DirectRepositoryWriter) error {
+	return rep.ContentManager().SyncMetadataCache(ctx)
 }
 
 func init() {
-	cacheSyncCommand.Action(directRepositoryAction(runCacheSyncCommand))
+	cacheSyncCommand.Action(directRepositoryWriteAction(runCacheSyncCommand))
 }