Sporadic test failure of `broadcast_commit_only_if_newer`.

[romanb]

Observed in the context of #100 but also affecting master. The witness:

thread 'voter::tests::broadcast_commit_only_if_newer' panicked at 'assertion failed: res.is_err()', src/voter/mod.rs:1118:6
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
test voter::tests::broadcast_commit_only_if_newer ... FAILED