Make sure promise::get_gid() can be called more than once

[hkaiser]

No description provided.

[hkaiser]

@Finomnis This should fix your problem, please verify

[hkaiser]

@Finomnis can this be merged now?

[Finomnis]

Hey :) my work cluster is/was down, so I wasn't able to verify. But if the test succeeds, it should work :)

[hkaiser]

@Finomnis Please let us know once you've tested it.

[Finomnis]

Confirmed. Fixed my problem.

[Finomnis]

I think we should revert this change. I can prove that there is no scenario where this would not cause an out-of-scope problem.
If someone needs to request a second id_type directly from a promise, this means that the first id_type he requested is out-of-reach for the person requesting it, and it can not be guaranteed that it did not get released yet.

example (pseudocode):

promise p;
function_one(p.get_gid());
function_two(p.get_gid()); // UNSAFE - function_one could have release its id_type already

this code can get re-arranged in different ways, but it always comes back to the problem that you need to keep a copy of the previous id_type to ensure that it does not get out of scope. like this:

promise p;
id_type id1 = p.get_gid();
function_one(id1);
id_type id2 = p.get_gid();
function_two(id2); // SAFE - id1 is still alive and ensures that agas did not delete the object yet

but this always leads to the scenario that the caller still has a copy of the id_type, which he can simply reuse instead of querying a new one:

promise p;
id_type id1 = p.get_gid();
function_one(id1);
function_two(id1); // SAFE

Therefore, I think we should revert this change as every single use-case it is actually needed does introduce a systematic error.