As mentioned in issue [#1560](https://github.com/restic/restic/pull/1560#issuecomment-364689346)
this changes the signature for `backend.Save()`. It now takes a
parameter of interface type `RewindReader`, so that the backend
implementations or our `RetryBackend` middleware can reset the reader to
the beginning and then retry an upload operation.
The `RewindReader` interface also provides a `Length()` method, which is
used in the backend to get the size of the data to be saved. This
removes several ugly hacks we had to do to pull the size back out of the
`io.Reader` passed to `Save()` before. In the `s3` and `rest` backend
this is actively used.
This pulls the header reads into a function that works in terms of the
number of records requested. This preserves the existing logic of
initially reading 15 records and then falling back if that fails.
In the event of a header with more than 15 records, it will read all
records, including the already-seen final 15 records.
Load pack header length and 15 header entries with single backend
request. This eliminates separate header Load() request for most pack
files and significantly improves index.New() performance.
Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
