Fast single-partition "Ephemeral Reads"

Introduce a special kind of non-durable read that provides only per-key linearizable isolation; i.e. strict-serializable isolation for single partition-key reads.
This read creates only a happens-before edge, by collecting dependencies for execution and ensuring that execution happens strictly after these dependencies
have executed, but at no precise time otherwise. So later writes may be witnessed, and if multiple keys are read they may represents different points in time.

patch by Benedict; reviewed by Ariel Weisberg for CASSANDRA-19305
52 files changed