Werk #13838: TCP fetcher: don't connect if no data is needed

When fetching the data from a Checkmk agent (unix/windows), the fetcher would establish a connection before checking the cache file. If the cached data was still valid the connection has been closed.

These "unused" connections could lead to unwanted agent executions, which in turn can lead to lost logwatch messages or a flapping hardware/software inventory.

In Checkmk 2.1 it additionally lead to irritating errors in the log of cmk-agent-ctl on the monitored host, such as "Request failed: tls handshake eof" or "Request failed: Connection reset by peer (os error 104)".

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