Refactor lib.TagSet to use a bitmask/bitset

[na--]

Probably not a huge deal in terms of performance, but the TagSet type would be much better if implemented as a bitmask/bitset (or even a plain old struct with bool fields) instead of the current map[string]bool.
We know all of the possible system tag values at compile time and we check the enabled values every time we build the tags for a metric, which would be much quicker without all of the map lookups. And making the system tag IDs in code be constants (with 1 << iota) instead of strings would probably reduce the probability of human errors when dealing with them. The only time we should use the string IDs is when we parse the user-given configuration (or inject the consolidated configuration back into the script).

[na--]

As an aside, while the bit mask can be trivially implemented by iotas, at least until we have more than 64 flags, the marshaling and unmarshaling of string values should probably happen with some sort of a go-generated code, like how we use enumer for the response types and the compression types.