…instead of starting new threads every time

globals are supposed to be upper case

this will better protect the user from wrong usage

during test_bootstrap_retries_fail() there is an exception raised (by
design): TelegramError('test')

For a reason I haven't managed to pinpoint the above exception in its
precise timing caused the Updater to be left in a state which is
'self.running == False', but the dispatcher threads already initialized.
This patch identifies this extreme case and makes sure to go over the
stop procedure.

even for the first unitest, it might come after another unitests from
another file which had already init the conpool.

telegram servers might send a reponse with HTTP 302 (redirect) to
another hostname. in such case HTTPSConnectionPool will fail to do the
job

…ram-bot into urllib3