switch from hGetSome to hGet

This should be essentially no-op change for hGetContentsMetered, since it
always gets the entire contents. So the only difference is that each chunk
of the lazy bytestring will always be the full chunk size. So, I'm pretty
sure this is safe. Also, the only current users of hGetContentsMetered are
reading files, so the stream won't block for long in the middle.

The improvement is that hGetUntilMetered will always get some multiple of
the defaultChunkSize. This will allow the S3 multipart code to pick a fixed
size and know that hGetUntilMetered will really get that size.

(cherry picked from commit bd09046291a5fde26afc02f5838c6594c1fa9c00)
update
Merge branch 'master' of ssh://git-annex.branchable.com