async_hooks: avoid decrementing iterator after erase

decrementing an iterator returned by std::vector::erase may have undefined behaviour and should not be used. Decrementing end() on an empty container is undefined and .erase() could leave the container empty.

Instead, by calling vec.erase(it--) we decrement the valid iterator before the erase operation but after being passed to the erase method.

In case of AsyncHooks::RemoveContext perform the cleanup of empty contexts upfront using std::remove_if because the iteration gets interrupted as soon as the context to be removed has been found.