Add config option for tcp keep-alive in intercept.v1, host.v1 and host.v2

[plorenz]

Add config option for tcp keep-alive in intercept.v1, host.v1 and host.v2. This should us to detect when a connection goes down, so we can tear down the associated ziti circuit. When there's a clean TCP close, we shouldn't need it, but this will help us in those cases where we have a non-graceful connection termination on the remote side of the connection. Additionally, users may want to be able to enable this behavior.

Intercept.v1 example:

{
    "addresses" : [ "echo.ziti" ],
    "portRanges" : [
        { "low" : 1234, "high" : 1234 }
    ],
    "protocols" : ["tcp"],
    "tcpOptions" : {
        "keepAlivePeriod" : "15s"
    }
}

Host.v1 example

{
    "address" : "localhost",
    "port" : 1234,
    "protocol" : "tcp",
    "tcpOptions" : {
        "keepAlivePeriod" : "15s"
    }
}

Host.v2 will use the same syntax as host.v1

[plorenz]

For now we're not going to make this configurable.