This refactor is to make it easier to see how serf feature flags are
encoded as serf tags, and where those feature flags are read.
- use constants for both the prefix and feature flag name. A constant
makes it much easier for an IDE to locate the read and write location.
- isolate the feature-flag encoding logic in the metadata package, so
that the feature flag prefix can be unexported. Only expose a function
for encoding the flags into tags. This logic is now next to the logic
which reads the tags.
- remove the duplicate `addEnterpriseSerfTags` functions. Both Client
and Server structs had the same implementation. And neither
implementation needed the method receiver.
And fix the 'value not used' issues.
Many of these are not bugs, but a few are tests not checking errors, and
one appears to be a missed error in non-test code.
