[xfrout-conf] configure data sources from data_sources module.

the configuration is maintained using the DataSrcClientsMgr helper class.
[xfrout-conf] use DataSrcClientsMgr in XfroutSession and its helper.

some stuff specific to the sqlite3 setup is now unnecessary and removed.
test cases are adjusted accordingly.
Merge pull request #47 from bundy-dns/xfrout-conf

use general data source conf for xfrout