Reuse ZeroMQChannels

[jacksontj]

Cache the sreq inside the zmqchannel instead of caching the entire channel object.

This is an improvement over #10332 in that you don't have to worry about someone creating a ZeroMQChannel then forking (thereby breaking the per-thread, per-pid rules). This way once it moves it will create a new sreq as needed.

[ghost]

Test PASSed.
Refer to this link for build results: http://jenkins.saltstack.com/job/salt-pr-build/4767/

[jacksontj]

Accidentally force pushed this, i'll update this again from my tower at home

[ghost]

Test Failed.

If the failures are unrelated to your code, don't stress, a core developer will know these apart.
In the future, if possible, please branch off a passing commit to avoid false positives.
Refer to this link for build results: http://jenkins.saltstack.com/job/salt-pr-build/4901/

[thatch45]

I hate it when @github does this, makes it look like I merged something that had no commits from a force push!