Improve sync source selection logging

Improve log verbosity around sync source selection to include reasons why a certain node is/isn't chosen as a sync source. For example, customers want to know why the "nearest" node isn't chosen (e.g. not being a voter) when initialSyncSourceReadPreference is "nearest".

FWIW, currently in the function _isEligibleSyncSource, we have some debug log messages (e.g. "Cannot select sync source because we are a voter and it is not"). I wonder if it would be good enough to make those info level logs.