Insert Learner/Voter should not panic.

[Hoverbear]

Currently these two functions have no return, yet they panic on an otherwise reasonable mistake which could be handled by application code.

https://github.com/pingcap/raft-rs/blob/1e0741a9aea3b242d33dadc46245ad611e7e7aa4/src/progress.rs#L104-L120

This issue would be solved with them returning a Result<()> with an error condition stating the voter/learner is already present, or some other #[must_use] value so that the user knows they need to handle this return value.

[Hoverbear]

This is resolved now. #102