Make DestructorGuard inherit from a base

Make DestructorGuard inherit from a base

Summary: There are more use cases that the Destruction/Guard pattern can be
used than current DelayedDestruction provides. This diff makes the pattern more
general (remove self destruct) and lets DelayedDestruction derive from that.
The functionalities of DelayedDestruction is kept unchanged.

I leave destroy(), Destructor class, and destroyPending_ in DelayedDestruction
since they are not required by our CallbackGuard in proxygen.

I add a shouldDestruct() function to allow customized conditions on when to
call destructor.

I haven't made destroyNow() a std::function since I only need it to be set at
instatiation time. If there is any other use case that needs destroyNow() to be
a std::function, I can do that as well.

Reviewed By: @afrind

Differential Revision: D2202575