libstore: exponential backoff for downloads

Closes #932

`connect-timeout` gets replaced by an exponential backoff for the
download timeout where the initial value is controlled by the setting
`initial-connect-timeout`.

Per iteration, the upper limit of the timeout is increased set to

    timeout := min(max_connect_timeout, initial_connect_timeout * 2^i)

I decided to move the entire timeout / tracking of attempts into its own
class to not make the filetransfer implementation more complex. Also,
that allows us to write unit-tests for it.

Setting `--download-attempts` to `0` is forbidden now and an exception
will be thrown. For `--offline` we set it to `1`, the behavior is
equivalent to what it was before: whether the max tries were exceeded is
only checked after the first download exception got thrown, i.e. there's
still one attempt being made.

The end-result - with timeouts being caused by a wrongly set proxy -
looks like this:

    $ env HTTPS_PROXY=1.1.1.1 nix store ping --store https://example.com
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 5006 milliseconds (curl error code=28); retrying in 422ms ms (attempt 1/5)
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 10010 milliseconds (curl error code=28); retrying in 1003ms ms (attempt 2/5)
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 20020 milliseconds (curl error code=28); retrying in 2018ms ms (attempt 3/5)
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 40007 milliseconds (curl error code=28); retrying in 4087ms ms (attempt 4/5)
    error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 80074 milliseconds (curl error code=28)

Change-Id: I9e8d08d78275bcf60080d663febc9e075243d36b

doc/manual/rl-next/exponential-backoff.md

@@ -0,0 +1,12 @@
+---
+synopsis: "libstore: exponential backoff for downloads"
+issues: [lix#932]
+cls: [3856]
+category: Fixes
+credits: [ma27]
+---
+
+The connection timeout when downloading from e.g. a binary cache is exponentially
+increased per failure. The option `connect-timeout` is now an alias to `max-connect-timeout`
+which is the maximum value for a timeout. The start value is controlled
+by `initial-connect-timeout` which is `5` by default.