This is a very large change which turns the post Config function in
backends into a state based call and response system so that
alternative user interfaces can be added.
The existing config logic has been converted, but it is quite
complicated and folloup commits will likely be needed to fix it!
Follow up commits will add a command line and API based way of using
this configuration system.
v1.4.6 of uplink allows us to do a negative offset from the end of the
file. This removes a round trip when requesting the last N bytes of a
file.
Previous to v1.4.6 of uplink it wasn't possible to do a negative offset
on download. This meant that to fulfill the semantics of http range
headers it was necessary to first fetch the size of the object via a
stat call and compute absolute offset and length.
This adds a context.Context parameter to NewFs and related calls.
This is necessary as part of reading config from the context -
backends need to be able to read the global config.
This provides two things:
* It gives Storj insight into which uplink clients are using the
network.
* It facilitate rclone participating in the Tardigrade Open Source
Partner Program https://tardigrade.io/partner/
