In order to keep connections up to date and dynamically discover
newly appeared hosts clients should time after time
('zmq_target_expire' period) poll redis and update
its socket connections. It should be done in conjuction
with target records TTL, because permanent polling
or per-each-send polling is very bad for performance.
Change-Id: I643a111fca8bac32f41ced232d54ff2a2ebcbf77
Closes-Bug: #1521178