* Load specified [validators_file] relative to config dir
* Add default [validators_file] to rippled-example.cfg
* Remove [validators] and [validation_quorum] from rippled-example.cfg
* Add [validation_quorum] to validators-example.txt
* Allow validators.txt to be a symlink
* Throw for invalid [validators_file] instead of logging
* Trust own master public key from configured manifest
* Do not load untrusted manifests from database
Trusted validators are loaded from [validators] and [validator_keys]
sections from both rippled.cfg and validators.txt
Quorum is loaded from [validation_quorum] section in validators.txt
only if it is not configured in rippled.cfg
