Rename GP parallel words to CBDB parallel.

[avamingli]

We will introduce Parallel Query in later talks. To make users clear that Parallel is a CBDB feature, not GPDB's
rename all of GP parallel words to CBDB.

GP don't have parallel feature, it would be more
appropriate to rename to our CBDB parallel words.

Rename gp_parallel files to cbdb_parallel files
and gp parallel functions.

Authored-by: Zhang Mingli avamingli@gmail.com

fix #ISSUE_Number

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[tuhaihe]

LGTM.