We decided to use our own map data structure in general for better
readability and performance.
Reviewers: sergey
Differential Revision: https://developer.blender.org/D7987
Allows to speed up lookups like "is property FOO of data BAR animated".
Can be used to optimize object's visibility check, but also allows to
check animation on bones without too much of time penalty.
The cache is shared between both nodes and relations builder.
Currently is not used, just a boilerplate for an upcoming changes in
an actual logic.
