Minor fixes

[eigenraven]

Minor code tidying fixes for a bunch of compiler warnings:

• Adding default initializers to fields without one
• Remove padding by reordering members in a couple of structs
• A few trivial copy->move changes
• Use = defaulted constructors and destructors where appropriate
• Also fixes a flaky PTP test, it was modifying an atomic just after rather than before the barrier.
• PTP group race condition fix
• Attempt to make scheduler tests less unreliable under TSan: with tsan overheads, I think the executor was able to finish executing its dummy task before the scheduler was able to execute the next one, resulting in re-use where it normally wouldn't occur

This should fix all of the current TSan issues listed in #189

[Shillaker]

Awesome, LGTM 👍

[eigenraven]

@Shillaker can you merge? "The base branch restricts merging to authorized users"

[Shillaker]

@KubaSz think I've given you sufficient permissions now. Let me know if not.