Checkmk Werk 12948: REST API ETag handling

ID: 12948 Title: REST API ETag handling Component: Core & setup Level: 1 Class: Bug fix Version: 2.1.0i1 In 2.0.0p6, the viewing of a host through the REST API might fail due to missing "modification time" information on the host. This led to a HTTP 500 error. Also, the calculation of the ETag was sometimes based on stale cached values, resulting in subsequent HTTP 412 "precondition failed" errors. This werk fixes both of these bug.