Misc changes from arch board review meeting for python

[YijunXieMS]

• Unify the constructor parameters across languages
• Remove partition_count from the result of get_properties()
• Remove the from_* except for from_connection_string.  Use a different credentials object
• Auto_reconnect should be default=true (this should probably be hidden as well) - ditto for keep_alive_interval - it should be hidden
• In receiver, consumer_group is required - it should be optional (with a default value of $default)
• Consider removing open() in the receiver - just manage the link in .receive() - if not removing open(), then add it to other languages.
• Implement __repr() and __str() for the events that are received
• Support custom decoders on events that we can pass into json decoder
• Remove the receiver.subscribe() for preview - discuss with Johan about design before inclusion
• Rename partition to partition_id (to sync with other languages)

Consider hiding queue_messages() / send_pending_messages() - it's covered by send()

[YijunXieMS]

All done except the the following two deferred items.

• repr()
• "Support custom decoders on events that we can pass into json decoder"

Two separate issues were created for them.