Slave not parsing protocol.X from slave.conf

[hrxcodes]

Describe the bug
It seems that slave process does not actually parses slave.conf protocol. options.

To Reproduce
Steps to reproduce the behavior:

1. Remove protocol.2=TLSv1.3 from slave config
2. Transfer something, observe TLSv1,3 being used in logs

Expected behavior
Protocol option to be parsed and respected. _sslProtocols exists in Slave.java but is unset. Parsing for master happens in https://github.com/drftpd-ng/drftpd/blob/master/src/core/master/src/main/java/org/drftpd/master/config/ConfigManager.java#L226

[tsautier]

@mvangoor @hrxcodes for review