[SPARK-12317][SQL] Support units (m,k,g) in SQLConf

This PR is continue from previous closed PR 10314.

In this PR, SHUFFLE_TARGET_POSTSHUFFLE_INPUT_SIZE will be taken memory string conventions as input.

For example, the user can now specify 10g for SHUFFLE_TARGET_POSTSHUFFLE_INPUT_SIZE in SQLConf file.

marmbrus srowen : Can you help review this code changes ? Thanks.

Author: Kevin Yu <qyu@us.ibm.com>

Closes #10629 from kevinyu98/spark-12317.