reenable pipelining via hashsum reordering support

Now that methods have the expected hashes available they can check if
the response from the server is what they expected. Pipelining is one of
those areas in which servers can mess up by not supporting it properly,
which forced us to disable it for the time being. Now, we check if
we got a response out of order, which we can not only use to disable
pipelining automatically for the next requests, but we can fix it up
just like the server responded in proper order for the current requests.

To ensure that this little trick works pipelining is only attempt if we
have hashsums for all the files in the chain which in theory reduces the
use of pipelining usage even on the many servers which work properly,
but in practice only the InRelease file (or similar such) will be
requested without a hashsum – and as it is the only file requested in
that stage it can't be pipelined even if we wanted to.

Some minor annoyances remain: The display of the progress we have
doesn't reflect this change, so it looks like the same package gets
downloaded multiple times while others aren't at all. Further more,
partial files are not supported in this recovery as the received data
was appended to the wrong file, so the hashsum doesn't match.

Both seem to be minor enough to reenable pipelining by default until
further notice through to test if it really solves the problem.
This therefore reverts commit 8221431757c775ee875a061b184b5f6f2330f928.

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