[SQL]Extract the joinkeys from join condition

Extract the join keys from equality conditions, that can be evaluated using equi-join.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #1190 from chenghao-intel/extract_join_keys and squashes the following commits:

4a1060a [Cheng Hao] Fix some of the small issues
ceb4924 [Cheng Hao] Remove the redundant pattern of join keys extraction
cec34e8 [Cheng Hao] Update the code style issues
dcc4584 [Cheng Hao] Extract the joinkeys from join condition