Feat/harmony cron

[snadrus]

Cron offers At() which schedules a task to be made that associates itself with an arbitrary table.
This is an unprivileged task: any task can do this.
Usage:
Fill your custom table with the custom values that matter to you. (returning rowID)
It should have an 'id' and 'task_id' column

deps.At(
time.Now().Add(3 * time.Hour),
"FooTask",
"tableFoo",
rowID)

2 fragile points:

1. it adds to harmony_task which beforehand only harmonytask did.
2. It required a PLSQL func to avoid the sqlInjection system complaining about an arbitrary table.

[snadrus]

Question for reviewer:
We currently take this low-effort work on all machines. Should we change CanAccept() to be pickier, or is it just not worth the hassle?

[snadrus]

This needs a DB-based unit test. We should hold until we have that framework up.