* [TD-5398]<fix>: taosdemo 4096 columns table creation failed.

* change column name prefix from col to c for maxium column number to fix buffer length.

* fix test case for column name be shorter.

* use accurate buffer length.

* remove space between columns and tags.

* change sql buffer to 1M align with taosdef.h.

* column buffer length be calculated.
Co-authored-by: NShuduo Sang <sdsang@taosdata.com>