Align the Scheduler DB directory structure for PostgresSchedDB with the rdbms model

stateDiagram-v2
    direction LR
    classDef whitesystem fill:white
    classDef needsreview fill:#ff8c1a
    scheduler:::whitesystem --> OStoreDB/:::whitesystem
    scheduler:::whitesystem --> PostgresSchedDB/:::needsreview
    scheduler:::whitesystem --> [...]:::whitesystem
    PostgresSchedDB/ --> schema/:::whitesystem
    PostgresSchedDB/ --> sql/:::needsreview
    PostgresSchedDB/ --> PostgresSchedDB.cpp[....]:::needsreview

changing to

stateDiagram-v2
    direction LR
    classDef whitesystem fill:white
    classDef allok fill:#66b3ff
    scheduler:::whitesystem --> OStoreDB/:::whitesystem
    scheduler:::whitesystem --> rdbms/:::allok
    scheduler:::whitesystem --> [...]:::whitesystem
    rdbms/ --> schema/:::whitesystem
    rdbms/ --> postgres/:::allok 
    rdbms/ --> SchedulerRDBMS.cpp[....]:::allok

and renaming all PostgresSchedDB classes and files to SchedulerRDBMS in order to later allow to extend the implementation to Oracle (we will need to add rdbms/interfaces and rdbms/oracle directories and adapt the implementation accordingly).