Potential server-side changes re terminology of master/slave.

As those changes become more concrete:

• client lib MUST: be aware of (i.e. work correctly with) any server-side changes to commands and return values, including any version-specific semantics
• client lib SHOULD: use preferred nomenclature where possible - happy to add new methods/enums etc and add [Obsolete(..., false)] to the old
• documentation SHOULD: use preferred nomenclature throughout (perhaps with "replica (historically: 'slave')" clarifications)

see https://github.com/antirez/redis/issues/5335

open question: does this impact the "no more 1.* builds" decision?