util/disk_cache: Add new mesa-db cache type

Introduce new cache type, the Mesa-DB. This is a single-file read/write
cache that is based on the read-only Fossilize DB cache. Mesa-DB supports
cache size capping. It's a much more efficient cache than the multi-file
cache because Mesa-DB doesn't have the inode overhead. The plan is to make
Mesa-DB the default cache implementation once it will be deemed as stable
and well tested. For now users have to set the new MESA_DISK_CACHE_DATABASE
environment variable in order to active the Mesa-DB cache.

Mesa-DB cache is resilient to corrupted cache files and doesn't require
maintenance from users and developers. The size capping is implemented
by evicting least recently used cache items and compacting the cache
database files with the evicted entries. In order to prevent frequent
compaction of the cache, at minimum a half of cache is evicted when cache
is full.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16888>

docs/relnotes/new_features.txt

@@ -1,2 +1,3 @@
 GL_ARB_shader_clock on llvmpipe
 VK_KHR_shader_clock on lavapipe
+Mesa-DB, the new single file cache type