Significantly reworked the monitoring infrastructure with a more granular per socket API and to play well with monitoring endpoints in application threads

The patch is meant to make the code easier to understand.
The 'wait' attribute is replaced by 'delayed_start'
and 'timer_started' attributes. The former is constant and
is initialized in the constructor. The latter is a flag
reflecting whether a timer has been started and changes during
the lifetime of the object.

This reverts commit dde69fb9.

This reverts commit 7b105865, reversing
changes made to 33459029.

Once the object has been terminated, it is unsafe for this object
to refer to its parent.

The bug was responsible for occasional
test_shutdown_stress failures.

This allows us to actually report an error to the caller on resolve
failure, rather than asserting later on in the io thread.
Signed-off-by: Staffan Gimåker <staffan@spotify.com>

Signed-off-by: Ivo Danihelka <ivo@danihelka.net>

Older versions of gcc have problems with in-line forward declarations
when there's a naming conflict with a global symbol.
Signed-off-by: AJ Lewis <aj.lewis@quantum.com>

Expand the original patch to all such forward declarations.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

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

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

This is a preliminary patch allowing for socket-type-specific
functionality in the I/O thread. For example, message format
can be checked asynchronously and misbehaved connections dropped
straight away.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

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

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

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

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

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

zmq_engine and tcp_socket merged into tcp_engine
zmq_connecter and tcp_connecter merged into tcp_connecter
zmq_listener and tcp_listener merged into tcp_listener
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

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

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

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