[SPARK-8756] [SQL] Keep cached information and avoid re-calculating footers in ParquetRelation2

JIRA: https://issues.apache.org/jira/browse/SPARK-8756

Currently, in ParquetRelation2, footers are re-read every time refresh() is called. But we can check if it is possibly changed before we do the reading because reading all footers will be expensive when there are too many partitions. This pr fixes this by keeping some cached information to check it.

Author: Liang-Chi Hsieh <viirya@appier.com>

Closes #7154 from viirya/cached_footer_parquet_relation and squashes the following commits:

92e9347 [Liang-Chi Hsieh] Fix indentation.
ae0ec64 [Liang-Chi Hsieh] Fix wrong assignment.
c8fdfb7 [Liang-Chi Hsieh] Fix it.
a52b6d1 [Liang-Chi Hsieh] For comments.
c2a2420 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into cached_footer_parquet_relation
fa5458f [Liang-Chi Hsieh] Use Map to cache FileStatus and do merging previously loaded schema and newly loaded one.
6ae0911 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into cached_footer_parquet_relation
21bbdec [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into cached_footer_parquet_relation
12a0ed9 [Liang-Chi Hsieh] Add check of FileStatus's modification time.
186429d [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into cached_footer_parquet_relation
0ef8caf [Liang-Chi Hsieh] Keep cached information and avoid re-calculating footers.