_artifactcache/artifactcache.py: Sealing away some the estimated size

Previously, the API contract was to expose the estimated_size variable
on the ArtifactCache instance for all to see, however it is only relevant
to the ArtifactCache abstract class code. Subclasses were informed to
update the estimated_size variable in their calculate_cache_size()
implementation.

To untangle this and hide away the estimated size, this commit
does the following:

  o Introduces ArtifactCache.compute_cache_size() API for external
    callers

  o ArtifactCache.compute_cache_size() calls the abstract method
    for the CasCache subclass to implement

  o ArtifactCache.compute_cache_size() updates the private
    estimated_size variable

  o All direct callers to ArtifactCache.calculate_cache_size(),
    have been updated to use the ArtifactCache.compute_cache_size()
    method instead, which takes care of updating anything local
    to the ArtifactCache abstract class code (the estimated_size)