Check membership before transferring leadership

[anhanhnguyen]

Hi,

During the testing with the current ra library, we noticed that leadership can be transferred to a non-voter member. We think non-voter members should not hold leadership.

Proposed Changes

• Added a membership check to ensure the target ServerId is a voter member.
• If the target is not a voter, return a non_voter_member error.

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[kjnilsson]

Sounds reasonable. Cheers.

[kjnilsson]

make the error {error, non_voter} please

[kjnilsson]

Also please update the type spec fr the transfer_leadership function in the ra module and perhaps the docs also.

[anhanhnguyen]

I've updated the docs, but I think the function spec does not require any changes. The {error, non_voter} is already covered under {error, term()} in ra:transfer_leadership/2 function specification.