1) Ensure web requests request data in-order by fixing the piecepicker
   to correctly use 'CanRequestMorePieces'

2) Fix a bug in the handling of padding files. A request for some data
   from a webseed needs to take padding files into account in all cases.
   A request can begin or end inside padding, it can also begin/end in
   regular files while optionally crossing one stretch of padding. This
   means the existing logic to insert synethic requests to the padding
   file are broken in a number of cases.

Running all the tests in parallel sometimes results in port collisions,
so use a wider range of random ports.

…sts into adjacent chunks before requesting

Use the original logic to detect when non-sequential requests are made,
and raise an error instead of handling it gracefully.

By default blocks are effectively random, and this is a bad access
pattern for many web servers. The piece picker should be requesting a
bunch of sequential blocks, which should be converted into a single
large http request, which (in turn) should be better for the http server
and also the library.

Add validation http requests