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ARROW-5576: [C++] Query ASF mirror system for URL and use when downloading Thrift

This also allows CHECKSUM values to be put in cpp/thirdparty/versions.txt for security purposes. Apache Thrift is still using MD5 for some reason, so we will need to fix that once they get their next release out (hopefully with SHA256 checksums)

Author: Wes McKinney <wesm+git@apache.org>

Closes #4558 from wesm/ARROW-5576 and squashes the following commits:

4700af407 <Wes McKinney> Disable log suppression in thrift_ep
bfbfbec8d <Wes McKinney> Query ASF mirror system for URL and use when downloading Thrift