Remove unnecessary forward declarations in RadosFs
Adapt the dir cache to add priority and limits

Which means that the dir cache is no longer just a list of dir cache
objects but it takes into account the priority (recently accessed
caches are kept for longer) and limits (part of the cache is
flushed when reaching a configured limit).
Update dir cache from RadosFsDir
Add the setter and getter methods for the dir cache size
Remove cache when a directory is deleted

This implied adding some helper methods for removing the from
RadosFsDir.
Do not update the directory contents after removing it

Instead remove its cache.
Don't set dir path if it's equal to the current one
Reset the cache pointer when setting the dir's path
test: Make RadosFsTest a friend class of RadosFs

This will allow testing RadosFs's internal logic.
test: Add unit-test for the dir cache of RadosFs