Feature/616 len of fiware service string

### Description
Changed max length of Fiware-Service (tenant) from 20 to 50 bytes.
Also put a maximum of 13 bytes for the database name (CLI-option '-db').

Mabye a little bit cryptic? Perphas we should add a small comment here, something like this:

"// eoo() checking needed to avoid that a case of "field not found" result in a assertion fault crash"