Implement metasearch for SRU endpoints
- primarily for OPAC catalogues in MWW
(from redmine: issue id 1323, created on 2019-09-04tgradl)
- Relations:
- relates #1026
- child #1322
- child #1324
- child #1325
- child #1326
- child #1327
(from redmine: issue id 1323, created on 2019-09-04tgradl)