Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

This means that all the handshaking while accepting incoming
connection is done exclusively in I/O threads, thus it won't
overload the application thread's mailbox.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

    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>