v2.0.0a6

What's Changed

• namespace version bump by @sean-parent in #541

• Bumping the inline namespace version to v2 to avoid ODR violations.
• The pre_exit code remains at v1 (it must be bumped more carefully).
• The portable default executor is moving to v2 - which may lead to more than one thread pool for the portable instance. Pinning the version here is more difficult.
• Removed dead reset() operations in future shared_base
• Documented (and assert) precondition that future::exception() is only invoked on a ready exception.
• Made more of the tuple meta-facilities public to avoid detail:: namespace usage in unit tests.
• Restructured unit tests to avoid sleep-wait-loops.

Full Changelog: v2.0.0a5...v2.0.0a6