Decrease the default 'TcpKeepAliveTime' for SDK3

Currently the value for 'TcpKeepAliveTime' (as covered on https://docs.couchbase.com/dotnet-sdk/2.7/client-settings.html#dotnet-clientconfiguration-settings) is 2 hours.

This is a very long time and in some environments this is not a very sensible default; for example, for Azure all external connections have a tcp timeout of 4 minutes.
Clearly the .NET SDK connections will not start triggering the tcp keepalives before this timeout kicks in.

I guess the 2 hours was taken from the Windows default, but given that the interval we set TCP keep alives (after they start) is 1000ms, I think it would make sense to decrease the initial window to something much smaller, perhaps a minute or two at most?

Given this is a breaking change (as some people may have been relying on the old default) perhaps we can consider this for SDK 3?