make parallel execution possible

[Flyingmana]

should already work during the cron, as only the current executed Index gets a lock, so the others are not locked. Means we could run different Indices at the same time.

Implementation Ideas:

1:
add an additional parameter to the shell script for process code, so its easy as executiong two shell
scripts at the same time.

2:
use threading/forking offered by php extensions to execute all indices in parallel on the same call