This is supposed to become part of the ZMTP/1.1.

The main differences from the ZMTP/1.0 framing protocol are:
- flags field comes first, followed by the length field
- long messages are signaled using a flag rather then 0xff escape
- length field does not include the flags field, 0 is a valid value

This patch introduces i_msg_sink and i_msg_source interfaces. This
allows us to make message encoder and decoder more general.

This patch fixes a bug in the message encoder which was
responsible for computing incorrect message offset.
The bug affected PGM receiver making it unable to
decode inital messages.

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 patch adds support for checking messages as they arrive
(as opposed to when they are recv'd by the user) and drop
the connection if they are malformed.

It also uses this new feature to check for validity of inbound
messages in REQ socket.
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>

Removal of ZMQ_IDENTITY resulted in various session classes doing
almost the same thing. This patch merges the classes into a single
class.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

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

So far there was no distinction between message parts used by 0MQ
and message parts used by user. Now, the message parts used by 0MQ
are marked as 'LABEL'.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

This patch addresses serveral issues:
1. It gathers message related functionality scattered over whole
   codebase into a single class.
2. It makes zmq_msg_t an opaque datatype. Internals of the class
   don't pollute zmq.h header file.
3. zmq_msg_t size decreases from 48 to 32 bytes. That saves ~33%
   of memory in scenarios with large amount of small messages.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

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

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

Sockets may now be migrated between OS threads; sockets may not be used by
more than one thread at any time. To migrate a socket to another thread the
caller must ensure that a full memory barrier is called before using the
socket from the target thread.

The new zmq_close() semantics implement the behaviour discussed at:

http://lists.zeromq.org/pipermail/zeromq-dev/2010-July/004244.html

Specifically, zmq_close() is now deterministic and while it still returns
immediately, it does not discard any data that may still be queued for
sending. Further, zmq_term() will now block until all outstanding data has
been sent.

TODO: Many bugs have been introduced, needs testing. Further, SO_LINGER or
an equivalent mechanism (possibly a configurable timeout to zmq_term())
needs to be implemented.

zmq_encoder/decoder are able to add/trim prefixes from messages; fair queueing and load balancing algorithms factorised into separate classes