more work on SRU explain filters

more work on SRU explain filters
let SRU explain packages pass through sru_z3950 filter
catch SRU explain packages in zeerex_explain filter
depreciate build_simple_explain() function
introduced build_sru_explain() function which does it all
introduced SRUServerInfo class used to determine SRU database, host and port _before_ parsing of SRU request did succeed, as one has to know which of multiple databases expain to send in case that SRU request is unparsable
still missing actual dump of XML dom explain from correct database
recognized that splitting SRU explain in zeerex_explain filter and SRU searchRetrieve in sru_z3950 filter is a bad idea, as double work is needed to parse the same explain XML dom info, but for sake of this checkin these are kept separated. planned to move SRU explain to sru_z3950 filter, and use zeerex_explain filter for z3950 packages zeerex explain operation only (maybe making mapping from explain classic to zeerex explain ??)