ReplicaSet support

Solves bug #493 

**NOT MERGE** before:

*  /etc/init.d/contextBroker has been fixed to take into account the new parameter BROKER_DATABASE_RPLSET
* Install & Admin manual describes the usage of new `-rplSet` CLI option and BROKER_DATABASE_RPLSET parameter
It could get more 'elegant' sending in a function pointer but, at the same time it would get more complex that way.
I see no reason to change this implementation.