ver-2.0.12.0

FIX BUGS:
[TD-2548]:wrong error message is showed when taos connect failed
[TD-2477]:taosd crash when executing queries
[TD-2538]: crash when sql has limit & offset
[TD-2534]: fix crash on null pointer parser
[TD-2528]: messy characters in alert msgs
[TD-2487]: create stble with a wrong type tag but report length too long
[TD-2464]: validate maxTablesPerVnode and reset to minTablesPerV…
[TD-2484]: use IPv4's AI_INET instead of AI_UNSPEC
[TD-2550]: fix nodejs bug in new version.
[TD-2482]: fix bug in the skiplist iteration during query processing.
[TD-2471]: fix the bug during the removal of elements in trashcan.
[TD-2488]: fix super table bug.

ENHANCEMENTS:
[TD-2434]: reduce the memory requirement during super table interval query.

NEW FEATURES:

DOCS:
[TECO-39]: continue refine connector page.

TEST:
[TD-2488]: add test case
[TD-965]: add test case for TWA