Reduce missing returns during reauth

When the master key rotates (on a schedule or via salt-key -d), minions
will not be able to decrypt messages from the master until reauth completes.

Therefore, new jobs will effectively be lost during the splay period.

This turns the reauth splay down to 10s (from 60s) and makes it a float
for more granularity.

Additinoally, this moves the 'running' function out of the saltutil exec
module and into salt.utils so it can be easily re-used. It's now wrapped
in the exec module.
Pylint
Merge pull request #15270 from cachedout/reauth

Reduce missing returns during reauth