Improve handling of block read request timeouts

Block read requests that take longer than the timeout results in an
`{error, disconnected}` tuple being returned instead of `{error,
timeout}`.  The current code is written such that when `n_val=1`
requests are enabled and a timeout occurs that returns `{error,
disconnected}`, the request is retried again with `n_val=1` set. This
means that the request is sent to the same vnode again. In the case of
a severely backed-up vnode queue this likely results in another
timeout and `{error, disconnected}` result from the pb client.  This
continues until the retry limit is reached and an eventual `{error,
notfound}` is returned to the process that called the block
server. Change the block server code to add `disconnected` as a reason
to trigger a read attempt that does not use the `n_val=1` option. This
should allow the read request to complete and not cause downloads to
stall in the case of one or more very slow responding Riak vnodes.
Merge branch 'bugfix/improve-disconnect-handling' into release/1.4