Add timeout for /is_synced endpoint in sidecar.py

[puppetninja]

In out prod environment we could observe that requests piling up as there is no timeout in default requests get method. Here we timeout and close the connection timely to avoid the piling up.

This is an alternative way to fix the issue mentioned here https://github.com/oxheadalpha/tezos-k8s/pull/562

[puppetninja]

please see my latest comment I have updated the comment with right probe filed name, please have a review

[harryttd]

thank you!