executor: use batch get to speed up LOAD DATA (pingcap#5632)

* use batch get to speed up LOAD DATA

server: optimize com_field_list, make use database faster. (pingcap#5677

) (pingcap#5679)

expression: refine the type infer of group_concat (pingcap#5582)

fix issue5462 (pingcap#5463) (pingcap#5492)

 *: improve performance of show variables (pingcap#5363)

* show: fix variable scope error (pingcap#5291)

* *: improve performance of show variables (pingcap#5297)

* *: session variable value should not be modified when get global variable (pingcap#5317)

* fix ci

* ddl,expression: make leak test more stable (pingcap#4895)

tidb: always rebuild plan for retry. (pingcap#5219)

Fixs record and index inconsistency bug.

release-1.0: cherry pick some commits (pingcap#5082)

expression: fix typo (pingcap#4958)

stats: fix estimation in index point query (pingcap#4959)