The hostname/username are now persisted when connecting to repository
in a local config file.
This prevents weird behavior changes when hostname is suddenly changed,
such as when moving between networks.
repo.Repository will now expose Hostname/Username properties which
are always guarnateed to be set, and are used throughout.
Removed --hostname/--username overrides when taking snapshot et.al.
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.
