fixed some issues that prevented go test from passing on Windows:
- webdav client used \ instead of /
- need retries around mmap.Open()
- paths are prefixed with C:\ on windows
- time.Now() does not always move forward on Windows
Previously, it was possible for Flush() to miss in-flight writes,
but only when using repository manually since Uploader guarantees
there are no in-flight writes when it completes.
With this change Flush() will guarantee that any pending writes
completed before Flush() has started are guaranteed to be committed
to the repository before Flush() returns.
This was actually a regression introduced in #105.
Added regression test to prevent it from reoccurring.
Also introduced strongly typed content.ID and manifest.ID (instead of string)
This aligns identifiers across all layers of repository:
blob.ID
content.ID
object.ID
manifest.ID
This updates the terminology everywhere - blocks become blobs and
`storage.Storage` becomes `blob.Storage`.
Also introduced blob.ID which is a specialized string type, that's
different from CABS block ID.
Also renamed CLI subcommands from `kopia storage` to `kopia blob`.
While at it introduced `block.ErrBlockNotFound` and
`object.ErrObjectNotFound` that do not leak from lower layers.
