[SPARK-8838] [SQL] Add config to enable/disable merging part-files when merging parquet schema

JIRA: https://issues.apache.org/jira/browse/SPARK-8838

Currently all part-files are merged when merging parquet schema. However, in case there are many part-files and we can make sure that all the part-files have the same schema as their summary file. If so, we provide a configuration to disable merging part-files when merging parquet schema.

In short, we need to merge parquet schema because different summary files may contain different schema. But the part-files are confirmed to have the same schema with summary files.

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

Closes #7238 from viirya/option_partfile_merge and squashes the following commits:

71d5b5f [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
8816f44 [Liang-Chi Hsieh] For comments.
dbc8e6b [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
afc2fa1 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
d4ed7e6 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
df43027 [Liang-Chi Hsieh] Get dataStatuses' partitions based on all paths.
4eb2f00 [Liang-Chi Hsieh] Use given parameter.
ea8f6e5 [Liang-Chi Hsieh] Correct the code comments.
a57be0e [Liang-Chi Hsieh] Merge part-files if there are no summary files.
47df981 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
4caf293 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into option_partfile_merge
0e734e0 [Liang-Chi Hsieh] Use correct API.
3b6be5b [Liang-Chi Hsieh] Fix key not found.
4bdd7e0 [Liang-Chi Hsieh] Don't read footer files if we can skip them.
8bbebcb [Liang-Chi Hsieh] Figure out how to test the config.
bbd4ce7 [Liang-Chi Hsieh] Add config to enable/disable merging part-files when merging parquet schema.