Add #include to string.h in blob.hpp

Required for memmove(), memcpy()
Add try_lock() to mutex_t
Add scoped_lock_t syntactic sugar
Declare ZMQ_CONFLATE option
Implement double buffer for conflate option

Add simple double buffer implementation tailored to handle msg_t,
i.e invoke msg_t::close instead of destructor and so on.
Seems to me mutex is good enough at this point.
Plug in dbuffer to serve the ZMQ_CONFLATE option

ZMQ_CONFLATE option is passed to pipepair() which creates a usual
ypipe_t or ypipe_conflate_t and plugs it into pipe_t under a common
abstract base.
Update doc for ZMQ_CONFLATE socket option
Fix indentation
Add test for ZMQ_CONFLATE option
Merge pull request #626 from dkrikun/master

Add ZMQ_CONFLATE socket option