Release 7.6.0: Added properties in smtp.properties, imap.properties & pop3.properties to explicitly choose SSL protocols

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Java7 has sporadic problems at SSL handshake, by choosing the protocol explicitly these problems can be avoided.

- New property com.openexchange.imap.ssl.protocols in file 'imap.properties'
- New property com.openexchange.smtp.ssl.protocols in file 'smtp.properties'
- New property com.openexchange.pop3.ssl.protocols in file 'pop3.properties'