We ran into a RuntimeError yesterday on this line, the callback can
modify this dict and make the iteration raise an error. Using keys
method to iterate is safer.
See
http://stackoverflow.com/questions/11941817/python-runtimeerror-dictionary-changed-size-during-iteration-how-to-avoid-th/11941855#11941855

Before this commit, protobuf and protobuf.socketrpc were used to serialize and
transmit messages between events clients. This change implements a simpler ZMQ
client/server events mechanism that uses ZMQ sockets for transmitting messages
from clients to server and to redistribute such messages to subscribed
clients.

Closes: #6359