Solves issue #2927

Problem: assertion failure in poller_base.cpp:42

execution

Solution: relax test assertion to what can be guaranteed

Solution: relaxed test assertions, based on the actual time passed,
instead of assuming that this equals to the time slept

load

Solution: add assertions, partially in debug build only, improved naming

* Problem: race conditions for options.linger

Solution: make options.linger atomic

* Problem: std::atomic not used on Visual C++ although it is available

Solution: change conditional compilation to recognize _MSC_VER

Add windows poll build on Appveyor

Problem: duplicated code, redundant member handle_valid, asymmetry between tcp_connecter and tcp_listener

Problem: code duplication and unnecessary nesting around ZMQ_THREAD_SAFE querying

querying

Solution: remove code duplication and rearrange conditions

Problem: code style issues in mechanism_t and socket_base_t

Problem: several fields are non-const without need

 Problem: unreachable code around socket_base_t::add/remove_signaler

fix Windows Intel compiler build failure

Solution: add appveyor job

  tcp_connecter and tcp_listener

Solution: remove duplication and redundant member, align handling of
handle in tcp_connecter and tcp_listener

Solution: use simpler operations, construct std::string only once

unnecessary construction of temporary std::string

Solution: removed duplication, removed construction of std::string

Solution: add const where easily possible

Solution: replaced by assertion

Problem: inconsistent formatting

Revert "Problem: Windows Intel compiler build broken"

Problem: Windows Intel compiler build broken

Solution: fix CMAKE_MODULES_PATH

Solution: fix execution directory

Solution: explicitly select clang-5.0

Solution: restrict to src, tests, perf, tools and include directories

Solution: add clang-format to cmake, and add another travis-ci build type

Solution: applied clang-format

Solution: added .clang-format file

Problem:  ZMQ_DISH triggers assert when hitting the watermark(?)

Solves issue #2907 by checking whether the msg already has a group

Improve handling of Jenkinsfile failed testlog archives

Port from zproject / czmq : Jenkinsfile :
* add RETRY_NUMBER suffix to test-suite tarballs
* and set build UNSTABLE if retries were needed

Problem: in case of exhausted resources on creation of a context, assertions are triggered

Solution: signal error to caller, and apply appropriate cleanup