pmemkv version 1.2

This release introduces a new experimental engine - csmap (concurrent sorted map, backed by libpmemobj-cpp's concurrent_map).
We have also provided support for custom comparators for sorted engines.

Features:

• csmap engine (concurrent sorted map)
• support for custom comparator for sorted engines

Optimizations:

• vsmap::put() now uses emplace() internally

Other changes:

• test framework refactor (scenario-based tests, extended tests for concurrency and sorting)