Merge pull request #499 from toxeus/apiMerge

OT_ME: merge called functions into caller
Dockerfile: don't ADD host build folders

`ADD .` will potentially add (at least one) `build` folder when used
in a CMake project. Since CMake `build` folders contain cached path
information a copy of it will make the compilation fail. Therefore,
we use `ADD` to explicitly select data needed for compilation and other
tests. The Docker image size is cut into half as a side effect.

Also, `rm -rf build` is removed since this command does not make any
sense to be executed in a folder that has just been created.
Removed unused message buffers in connection.
Removed linking to ZMQ in ext and added it for client and server libraries.
Merge pull request #500 from toxeus/fixDocker

Dockerfile: don't ADD host build folders
Dockerfile: add missing backslash

Before `cmake` was called without arguments because the backslash
was missing which is needed to indicate continuation lines in shell.

Also, CMake's `--debug-output` flag is removed because it's meant
for debugging and clutters CMake's output which leads to warning
messages being overlooked. Else, the missing backslash would have
been found sooner.
Dockerfile: fix Python3 include path
Merge pull request #495 from toxeus/socket_cleanup

Socket cleanup
Merge pull request #501 from toxeus/fixDocker

Dockerfile: fix Python bindings
docker use pre-build swig3.0
docker: avoid superfluous packages
Merge pull request #502 from mhamrle/docker

Update docker file
ZeroMQ: update C++ bindings

As a side effect, we can loosen up the ZMQ version check because
the C++ bindings update contains a fix for libzmq 4.1.x.
Merge pull request #503 from toxeus/socket_cleanup

ZeroMQ: update C++ bindings
OT_ME: merge called function into adjust_usage_credits()
OT_ME: merge called function into get_market_recent_trades()
OT_ME: merge called function into get_nym_market_offers()
OT_ME: merge called function into get_market_offers()
OT_ME: merge called function into get_market_list()