Fix for #133 use requests Session object with Retry capabilities

[justb4]

This fix should improve HTTP connection handling by GHC Probes significantly:

1. use requests Session objects to reuse connections (works via urllib3)
2. implement the golden tip from: https://www.peterbe.com/plog/best-practice-with-retries-with-requests . Within the Probe object a Session with a retry/backoff schema is created.
3. upgrade requests package to latest version 2.23.0

Especially in Probes where all N Layers of a WMS/Tiling endpoint are accessed/crawled this already gives a factor 3 performance improvement. In the old case a new connection was setup for each request.

The false negatives should be of the past. Will first observe for some time on the GHC demo server (where I get this stream of Failing/Fixed emails constantly...).

OPTION for later: may make this mechanism and its parameters configurable.

[justb4]

@tomkralidis @borrob merging anyway. Session also mimics user-browser behavior (keeping connections) Fully tested locally with both Docker and regular. Running on demo.geohealthcheck.org will give a more realistic scenario. (and see if the email stream is reduced).