Enumerate the possible featureCompatibilityVersion states in one atomic variable

There are two separate atomic variables for the version and targetVersion fields in serverGlobalParams. There is a potential for a races when reading the actual state through isFullyUpgradedTo36() and similar accessors, which must read both variables separately.

The ServerGlobalParams::FeatureCompatiblity::Version enum should enumerate the following states in one atomic variable. Getters and setters should be appropriately modified.