Simplify the variable expansion code. Instead of using a separate goroutine
and two channels, use a struct + a simple function call.
No functional changes.
Simplify the code for the two-way variables' value-to-variable replacement.
Instead of using a goroutine and two channels, use a separate struct and
call a function on that directly.
No functional changes.
Two-way variable implementation in the job submission end-point. Where
Flamenco v2 did the variable replacement in the add-on, this has now
been moved to the Manager itself. The only thing the add-on needs to
pass is its platform, so that the right values can be recognised.
This also implements two-way replacement when tasks are handed out, such
that the `{jobs}` value gets replaced to a value suitable for the
Worker's platform as well.
