1.0.0-dev.4

Fix heap corruption due to wrong usage of malloc.allocate

• Use periodic timer to poll sockets every second
• Poll all messages on socket instead of one for each event to not loose messages
• Reuse zeromq message pointer
• Improve return code handling
• Rename _isActive of ZContext to _shutdown
• Rename _handle and _zmq of ZSocket to _socket and _context
• Add stream for ZFrames to ZSocket
• Always show error code in ZeroMQException
• Fix pubspec of example