pmemstream version 0.2.1

This is, most likely, the last patch release of pmemstream project. It contains multiple fixes, documentation updates, refactors, minor CI/examples/CMake changes, along with new and updated tests.

The most relevant updates are listed below:

• Add shadow value to pmemstream_persisted_timestamp
• Fix and prune fences in atomic span ops
• Fix races (e.g., in region.c) and issues reported by TSAN
• Fix append benchmark for concurrent ops
• Extend entry_iterator_is_valid with an additional check
• Enable prefault at open/create time
• tests: extend multiple existing tests and add missing TCs
• tests: add check for recovery during recovery (a corner case for torn recovery process)
• Extend CMake with new options (e.g., for Thread Sanitizer and increased fortification level)
• Update manpages with detailed descriptions