Add support for SQL server cluster


Added an option to connection.config configuration file to specify datasource for readonly connections. Attribute name is "readInstanceConnectionString"  for element <connection>. 

The value is database connection string similar to connectionString attribute.

The parameter of the connection string tells SQL server cluster to use that connection for readonly slave instances in application with intent=readonly.

For example
readInstanceConnectionString="data source=testdb;integrated security=sspi;initial catalog=ag;application intent=readonly"

Readonly connection will be used by itembox queries in user interactive mode.