Support for remote content repository where all contents and
manifests are fetched over HTTP(S) instead of locally
manipulating blob storage
* server: implement content and manifest access APIs
* apiclient: moved Kopia API client to separate package
* content: exposed content.ValidatePrefix()
* manifest: added JSON serialization attributes to EntryMetadata
* repo: changed repo.Open() to return Repository instead of *DirectRepository
* repo: added apiServerRepository
* cli: added 'kopia repository connect server'
This sets up repository connection via the API server instead of
directly-manipulated storage.
* server: add support for specifying a list of usernames/password via --htpasswd-file
* tests: added API server repository E2E test
* server: only return manifests (policies and snapshots) belonging to authenticated user
This is mostly mechanical and changes how loggers are instantiated.
Logger is now associated with a context, passed around all methods,
(most methods had ctx, but had to add it in a few missing places).
By default Kopia does not produce any logs, but it can be overridden,
either locally for a nested context, by calling
ctx = logging.WithLogger(ctx, newLoggerFunc)
To override logs globally, call logging.SetDefaultLogger(newLoggerFunc)
This refactoring allowed removing dependency from Kopia repo
and go-logging library (the CLI still uses it, though).
It is now also possible to have all test methods emit logs using
t.Logf() so that they show up in failure reports, which should make
debugging of test failures suck less.
