1. ZMQ_LINGER option can be set/get
2. options are part of own_t base class rather than being declared
separately by individual objects
3. Linger option is propagated with "term" command so that the
newest value of it is used rather than the stored old one.
4. Session sets the linger timer if needed and terminates
as soon as it expires.
5. Corresponding documentation updated.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
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