Release 2.1.0

Both ptrack engine and public SQL API are now inside extension. Only
certain required hooks have been left as a PostgreSQL core patch.

Also many things have changed since 2.0.0:

• Fixed many compatibility bugs: alignment issues with atomics usage on Win32,
  with atomics simulation via spinlocks.
• Added versioning into the ptrack map.
• Added automated standalone testing with pg_probackup via travis-ci in
  different modes, including a 'legacy' mode, i.e. with EXEC_BACKEND
  and --disable-atomics.
• Removed pg_ptrack_get_block.

Upgrading from 2.0.0:

• Put shared_preload_libraries = 'ptrack' into postgresql.conf.
• Rename ptrack_map_size to ptrack.map_size.
• Do ALTER EXTENSION 'ptrack' UPDATE;.
• Restart server.