8.8 add support for INCREX

[mgravell]

INCREX is pre-checked INCRBY[FLOAT] with built in expiration support

Notes:

• there is a new ENX expiration concept; this is added into the existing Expiration type via a new ExpirationFlags
• new ENX is rejected by default on pre-existing Expiration scenarios; opt-in only
• core API is StringIncrement with a key, a value that is either long or double, Expiration (non-optional, but default is fine), and optional upper bound/lower-bound (in the same measure as the value). The result is a new StringIncrementResult<T> where T is the value kind (long/double).
• the new API is marked [Experimental] against 8.8, as are the new expiration flags scenario
• implemented with custom message / result processor
• unit tests implemented via toy-server
• integration tests exist but fail; feature is not merged