Some commands (such as the _configsvr commands and the setFeatureCompatibilityVersion command) must always use w:majority write concern. Right now different commands take different approaches to enforcing this. We should decide on a single strategy for this, ideally one that leverages the existing write concern handling code in dbcommands.cpp and returns write concern errors as 'writeConcernError' fields in the command response, in line with how the batch write commands report write concern errors