Use ReadFull instead of ioutil.ReadAll to read objects

[arodland]

ioutil.ReadAll uses a bytes.Buffer, which allocates memory by doubling.
Since we know exactly how muh data we expect to get, we can allocate it
in advance. This reduces the total amount of allocation, and ensures
that the slice stored in the item won't have excess capacity (which can
affect memory usage if the item is held for a long time, for instance in
a secondary cache).

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[arodland]

This passes tests; it would be nice to have a benchmark to show the difference in allocs, but none of the existing benchmark tests exercise the network read path, and I wasn't ambitious enough to write one that does. The difference is clearly visible in the heap profile of a real app, though.

[arodland]

CLA signed; hopefully the "alternate email" works as it should.

[googlebot]

CLAs look good, thanks!

[dwbuiten]

@bradfitz Ping on this, if you have time, of course; I'd love to not have to use an internal fork of this package in order to get highly reduced memory usage.

Sorry for the noise!