Lua stack and prefetch variables

[ethervoid]

We've found that with a great number of delayed messages, using the max LUA stack in the prefetch for delayed messages and multiple workers you can end up having stack overflow in Redis and crashing the LUA script so this PR intends to minimize that situation:

• Adding configurable environment variable for the delayed queue so we can define the maximum amount of delayed messages fetched
• Adding configurable environment variable to define the max LUA stack value
• Set the maximum unpacking value to 1/2 of the max LUA stack

Related to #433

[ethervoid]

@Bogdanp I've tried to run tox and pass the test but it's failing for me in Apple M1 and also in docker with ubuntu. Is there anything special I have to do?

I've been getting this error in Mac and Ubuntu inside docker:

py38-cpython installed: alabaster==0.7.12,attrs==21.2.0,Babel==2.9.1,backports.entry-points-selectable==1.1.0,bleach==4.1.0,bump2version==1.0.1,certifi==2021.10.8,charset-normalizer==2.0.7,colorama==0.4.4,coverage==6.0.2,distlib==0.3.3,docutils==0.17.1,dramatiq==1.11.0,filelock==3.3.0,flake8==4.0.1,gevent==21.8.0,greenlet==1.1.2,idna==3.2,imagesize==1.2.0,importlib-metadata==4.8.1,iniconfig==1.1.1,Jinja2==3.0.2,keyring==23.2.1,MarkupSafe==2.0.1,mccabe==0.6.1,packaging==21.0,pkginfo==1.7.1,platformdirs==2.4.0,pluggy==1.0.0,pockets==0.9.1,prometheus-client==0.11.0,py==1.10.0,py-cpuinfo==8.0.0,pycodestyle==2.8.0,pyflakes==2.4.0,pygal==2.4.0,pygaljs==1.0.2,Pygments==2.10.0,pyparsing==2.4.7,pytest==6.2.5,pytest-benchmark==3.4.1,pytest-cov==3.0.0,pytz==2021.3,readme-renderer==30.0,redis==3.5.3,requests==2.26.0,requests-toolbelt==0.9.1,rfc3986==1.5.0,six==1.16.0,snowballstemmer==2.1.0,Sphinx==1.7.9,sphinxcontrib-napoleon==0.7,sphinxcontrib-serializinghtml==1.1.5,sphinxcontrib-websupport==1.2.4,toml==0.10.2,tomli==1.2.1,tox==3.24.4,tqdm==4.62.3,twine==3.4.2,urllib3==1.26.7,virtualenv==20.8.1,watchdog==2.1.6,watchdog-gevent==0.1.1,webencodings==0.5.1,zipp==3.6.0,zope.event==4.5.0,zope.interface==5.4.0
py38-cpython run-test-pre: PYTHONHASHSEED='3419722698'
py38-cpython run-test: commands[0] | py.test --benchmark-skip
ImportError while loading conftest '/Users/ethervoid/development/dramatiq/tests/conftest.py'.
tests/conftest.py:7: in <module>
    import pylibmc
E   ModuleNotFoundError: No module named 'pylibmc'
ERROR: InvocationError for command /Users/ethervoid/development/dramatiq/.tox/py38-cpython/bin/py.test --benchmark-skip (exited with code 4)

and looks like tox is not installing the necessary dependencies. Anything I'm missing?

[Bogdanp]

Thanks! I've merged the change with rebase.

Re. tox, did you run it with a specific environment (eg. tox -e py38-cpython)? When you do that, it should automatically install everything.