The HTTPClient used in watcher is based on the apache http client. The
current client is using a lot of defaults - which are not always
optimal. Two of those defaults are the maximum number of total
connections and the maximum number of connections to a single route.

If one of those limits is reached, the HTTPClient waits for a connection
to be finished thus acting in a blocking fashion. In order to prevent
this when many requests are being executed, we increase the limit of
total connections as well as the connections per route (a route is
basically an endpoint, which also contains proxy information, not
containing an URL, just hosts).

On top of that an additional option has been made configurable to evict
long running connections, which can potentially be reused after some
time. As this requires an additional background thread, this required
some changes to ensure that the httpclient is closed properly. Also the
timeout for this can be configured.

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