Create zmq context once per driver instance

Now the zmq context is created per each socket.
Such approach is not optimal. Create zmq context once per
driver instance and pass it as an argument to ZmqSocket contstructor.

Change-Id: I2dfa338bd9ea124e2b20a2150a42fb9bbcc25cee
blueprint: https://blueprints.launchpad.net/oslo.messaging/+spec/zmq-context-per-driver-instance

specs/liberty/zmq-context-per-driver-instance.rst

@@ -0,0 +1,141 @@
+=========================================================
+ZeroMQ Context should be created once per driver instance
+=========================================================
+
+https://blueprints.launchpad.net/oslo.messaging/+spec/zmq-context-per-driver-instance
+
+ZeroMQ context object has singleton nature, so producing it
+more than once per driver is inefficient consuming of resources.
+
+Now it is created per socket.
+
+Proposed change
+===============
+
+oslo-messaging
+--------------
+
+Move creation of zmq context from ZmqSocket constructor
+:meth:`ZmqSocket.__init__`
+
+::
+
+   # oslo_messaging/_drivers/impl_zmq.py
+
+   class ZmqSocket(object):
+
+      def __init__(self, addr, zmq_type, bind=True, subscribe=None):
+         self.ctxt = zmq.Context(CONF.rpc_zmq_contexts)
+         self.sock = self.ctxt.socket(zmq_type)
+
+
+to ZmqDriver constructor :meth:`ZmqDriver.__init__`
+
+::
+
+   # oslo_messaging/_drivers/impl_zmq.py
+
+   class ZmqDriver(base.BaseDriver):
+
+       def __init__(self, conf, url, default_exchange=None,
+                    allowed_remote_exmods=None):
+       self.ctxt = zmq.Context(CONF.rpc_zmq_contexts)
+
+Update :meth:`ZmqSocket.__init__` to pass zmq context as an argument
+
+::
+
+   # oslo_messaging/_drivers/impl_zmq.py
+
+   class ZmqSocket(object):
+      ...
+      def __init__(self, zmq_ctx, addr, zmq_type, bind=True, subscribe=None):
+         self.ctxt = zmq_ctx
+         self.sock = self.ctxt.socket(zmq_type)
+
+
+Alternatives
+------------
+
+None
+
+Impact on Existing APIs
+-----------------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Configuration Impact
+--------------------
+
+None
+
+Developer Impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  Oleksii Zamiatin (ozamiatin@mirantis.com)
+
+Other contributors:
+  None
+
+Milestones
+----------
+
+next-kilo
+
+Work Items
+----------
+
+1. Perform the code movements
+2. Update all places in impl_zmq where socket used
+
+
+Incubation
+==========
+
+N/A
+
+Adoption
+--------
+
+N/A
+
+Library
+-------
+
+oslo.messaging
+
+Documentation Impact
+====================
+
+N/A
+
+Dependencies
+============
+
+None
+
+References
+==========
+
+.. note::
+
+   Check neutron https://bugs.launchpad.net/neutron/+bug/1364814