JIRA: https://issues.apache.org/jira/browse/SPARK-8359

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

Closes #6814 from viirya/fix_decimal2 and squashes the following commits:

071a757 [Liang-Chi Hsieh] Remove maximum precision and use MathContext.UNLIMITED.
df217d4 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into fix_decimal2
a43bfc3 [Liang-Chi Hsieh] Add MathContext with maximum supported precision.
72eeb3f [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into fix_decimal2
44c9348 [Liang-Chi Hsieh] Fix incorrect decimal precision after multiplication.