http: add Boost 1.49 compatibility
http: add Boost 1.49 compatibility

`try_join_for` was introduced in Boost 1.50:
http://www.boost.org/doc/libs/1_50_0/doc/html/thread/thread_management.html#thread.thread_management.thread.try_join_for

1.49 has `timed_join`, one can accomplish the same with:
http://www.boost.org/doc/libs/1_49_0/doc/html/thread/thread_management.html#thread.thread_management.thread.timed_join

However, `timed_join` was deprecated in 1.50. So a conditional is
necessary.

This solution was tested in #7031.

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