Release 6.20.0: Configured max. connection restriction for GMail by default

[changerequest]

GMail limits max. number of concurrent connection to 2. We have to obey that restriction in order to offer a stable access to a user's GMail account.

Therefore the pre-configured value for property com.openexchange.imap.maxNumExternalConnections is set to imap.gmail.com:2,imap.googlemail.com:2.