pmemstream version 0.2.0

This is the second (still minor) release of pmemstream project.

It contains many functional updates, API changes, and reactors, along with new tests and examples.

New features:

• Asynchronous API, which uses libminiasync (https://github.com/pmem/miniasync) for concurrently appending data to the stream; currently, multiple threads can append data concurrently to different regions, but no two threads can append to the same one.
  It includes new functions for appending and publishing data. It's exposing 2 stages of an append - committed and persisted with 2 functions pmemstream_async_wait_*(timestamp) to wait for the given entry to be available for reading and/or for persistency guarantee.
• Changed region and entry iterators API. It now provides seek_first, next, and is_valid functions for both types of iterators. It offers higher flexibility.
• Multiple regions. A simple region allocator, allowing creation of multiple regions; for now, only supports a single region size (within a given stream).
• Timestamps for each entry to provide global entries' order (and easier recovery).
• Exposed pmemstream_region_usable_size function to read the currently available size in a given region; it can be used with the combination of "entire region size" function (pmemstream_region_size) to, e.g., calculate the already used region's space.
• Full documentation of public API in the form of manpages.

Other changes:

• Rename pmemstream_entry_length to pmemstream_entry_size for consistent naming convention,
• Add a simple "append" benchmark,
• Enable integrity, consistency, and sanitizers tests,
• Add 4 more examples.