Workflow plugin causes memory leak, even in "empty" flow.

Error reproduction scenario:
1. Create empty workflow with simple command, i.e. echo "123". To reproduce error quicker, create more (I've created 10 workflows)
2. Each workflow has schedule "* * * * * *" (run every minute)
3. Measure memory for the process (I've used standard /proc/[pid]/statm data+stack values)

Results are listed in attachment (with (above) or without Discard Old Builds (below)).

In more complex scenario (with script loaded from git) it causes more massive memory leak (for me 2MB/build, due to lack of effective garbage collection).