Eliminate extra operations during metadata cache pruning

While doing performance testing for DRILL-4530 using a new data set and queries, we found two potential performance issues: (a) the metadata cache was being read twice in some cases and (b) the checking for directory modification time was being done twice, once as part of the first phase of directory-based pruning and subsequently after the second phase pruning. This check gets expensive for large number of directories. Creating this JIRA to track fixes for these issues.