Kafka support

[CountRedClaw]

Based on #600 but switched from pykafka to https://github.com/confluentinc/confluent-kafka-python as pykafka project was archived.
Also aiokafka was mentioned here, but I didn't find any usages of asyncio libraries in Kombu. Thus I assumed that the best option for me would be confluent-kafka-python.

Integration tests are green, but had to exclude BaseTimeToLive and BasePriority tests because Kafka does not allow to set expiration time per message, as well as it does not provide message priority feature (Kafka partition is an immutable ordered log of messages).

I'm not sure about the relevance of my PR (maybe you meant to use another library), so let me know if it makes sense.

[lgtm-com]

This pull request introduces 1 alert when merging 52fb571 into aab2588 - view on LGTM.com

new alerts:

• 1 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert when merging 39da6bb into aab2588 - view on LGTM.com

new alerts:

• 1 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert when merging 6c6acb5 into aab2588 - view on LGTM.com

new alerts:

• 1 for Missing call to `__init__` during object initialization

[matusvalo]

Can you add also a documentation to head of file - see redis transport as an example:

kombu/kombu/transport/redis.py

Lines 1 to 52 in aab2588

	"""Redis transport module for Kombu.
	Features
	========
	* Type: Virtual
	* Supports Direct: Yes
	* Supports Topic: Yes
	* Supports Fanout: Yes
	* Supports Priority: Yes
	* Supports TTL: No
	Connection String
	=================
	Connection string has the following format:
	.. code-block::
	redis://[USER:PASSWORD@]REDIS_ADDRESS[:PORT][/VIRTUALHOST]
	rediss://[USER:PASSWORD@]REDIS_ADDRESS[:PORT][/VIRTUALHOST]
	To use sentinel for dynamic Redis discovery,
	the connection string has following format:
	.. code-block::
	sentinel://[USER:PASSWORD@]SENTINEL_ADDRESS[:PORT]
	Transport Options
	=================
	* ``sep``
	* ``ack_emulation``: (bool) If set to True transport will
	simulate Acknowledge of AMQP protocol.
	* ``unacked_key``
	* ``unacked_index_key``
	* ``unacked_mutex_key``
	* ``unacked_mutex_expire``
	* ``visibility_timeout``
	* ``unacked_restore_limit``
	* ``fanout_prefix``
	* ``fanout_patterns``
	* ``global_keyprefix``: (str) The global key prefix to be prepended to all keys
	used by Kombu
	* ``socket_timeout``
	* ``socket_connect_timeout``
	* ``socket_keepalive``
	* ``socket_keepalive_options``
	* ``queue_order_strategy``
	* ``max_connections``
	* ``health_check_interval``
	* ``retry_on_timeout``
	* ``priority_steps``
	"""

[auvipy]

a rebase & docs update needed

[lgtm-com]

This pull request introduces 2 alerts when merging 3b7d903 into 14d395a - view on LGTM.com

new alerts:

• 1 for Unused import
• 1 for Missing call to `__init__` during object initialization

[lgtm-com]

This pull request introduces 1 alert when merging 9a1fe95 into 14d395a - view on LGTM.com

new alerts:

• 1 for Missing call to `__init__` during object initialization

[CountRedClaw]

Seems like confluent-kafka-python doesn't support PyPy. So had to disable PyPy tests for kafka.

[auvipy]

Seems like confluent-kafka-python doesn't support PyPy. So had to disable PyPy tests for kafka.

that's OK, we just need to mention it on docs/release notes

[lgtm-com]

This pull request introduces 1 alert when merging 6cec1e9 into 14d395a - view on LGTM.com

new alerts:

• 1 for Missing call to `__init__` during object initialization