Thread: provide physical concurrency conditional until it no regression is introduced.
Thread: future::then should join the thread before destruction.
Thread: provide physical_concurrency conditionally.
Thread: update history.
Thread: Remove obsolete GCC version check.
Thread: use boost::disable_if instead of disable_if to avoid ambiguity.
Thread: Added a polymorphic executor and an executor_adaptor.
Thread: try to fix issue with double definition of shared_state_base.
Revert "Thread: provide physical concurrency conditional until it no regression is introduced."

This reverts commit 271ba281aecd5c556b2b863a9f05a8066e717854.
thread: physical_concurrency - work around getline issue in libc++