Fix Empty Results in Certain Reverse Chaining FHIR Search Queries

[alexanderkiel]

For FHIR search queries like Patient?gender=male&_has:Encounter:patient:class=IMP were the reverse chaining parameter _has follows a normal parameter like gender, it can happen that the result is either empty or much less than expected.

The core problem is that the RocksDB iterator for the SearchParamValueResource index is used concurrently for finding the male patients and the inpatient encounters. In this situation, we have to use a second iterator here.

Normally the second search parameter uses only iterators over the ResourceSearchParamValue index. So this issue only happens on reverse chaining as second parameter.