rdbms: make schema/prefix handling more consistent in LoadBalancer

rdbms: make schema/prefix handling more consistent in LoadBalancer

Move "tablePrefix" logic to LoadBalancer::reallyOpenConnection and
make it more consistent for local vs foreign connections. The prefix
for local connections is now set even if $domain does not provide a
database.

Also set the schema even if $domain does not provide a database.

Typically a database domain is either fully specified or comes from
DatabaseDomain::newUnspecified(), but there's no reason not to use
at least the schema/prefix of otherwise unspecified domains. The only
other sensible option would be some sort of exception.

Change-Id: Ib52f4b69e429810e3c5eac92a48aecf7b22e86e6