rdbms: add and enforce DB_REPLICA/DB_MASTER roles in DBConnRef

Even if there is only one database, if a DB_REPLICA connection
reference is obtained and an attempt is made to write it then
it will still fail. This can make it easier to spot incorrect
database usage even in a simple vanilla developer environment.

Note that methods like ILoadBalancer::getConnectionRef() can
be used for local connections as well as foreign ones.

Change-Id: I5523daad1bdd64624d3e0dd41bfd8d078b1078b0