Check Google HTTP Client / Apache HttpClient versions at runtime and fail or warn if not using matching versions

[chanseokoh]

We've observed issues when not using the right versions, e.g., #1018, #1390, #1418. For these cases, they surfaced totally unrelated error messages that might have driven us to a completely wrong direction, and I honestly think in some cases, it was with some luck that we could suspect the actual cause.

Other recent related issues were behavioral changes in newer versions, such as #1517 and #1415 (comment). (BTW, in these cases, I was intentionally over-specific on the exception classes when I implemented the SSL failover support to err on the side of caution, so I was somewhat expecting these issues might surface in the future.)

Considering these, it sounds like a great feature that we dynamically check the versions of the loaded Google HTTP Client library and the Apache HttpClient library at runtime and have the build fail or display a warning with proper messages.