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').

Add a "type 2" trace (see https://forge.fi-ware.org/plugins/mediawiki/wiki/fiware/index.php/Publish/Subscribe_Broker_-_Orion_Context_Broker_-_Installation_and_Administration_Guide#Alarm_conditions) with the following message:

"Database Error (error retrieving _id field in doc: %s)"

where that `%s` resolves to `r.toString().c_str()`.