Moved the expression dump into its own visitor

- FdbQueryExpressionStringBuilder visits an FdbQueryExpression and tries to output a nicely formatted dump
- removed AppendDebugStatement(..) from FdbQueryExpression since it is not needed anymore
Removed uneeded custom Constant expression

- FdbDatabaseQuery and FdbIndexQuery are only starter objects, that cannot be executed, so they don't need to hold an expression
Removed unnecessary copy in ParameterRewriterVisitor constructor