rpc: fix deadlock when tsched queue is full and session limit reached

[TD-3155]<test>: add test case

[TD-3177] <fix>: fix demo.c checking result error mistake.

[TD-3170]taosd/taos crash when group by tag with type double and value NULL 

[TD-3165]taos crash when query bin/hex values

[TD-3171]<fix>: response error code if any

Update docs.md

Modify the chapter order

[td-225]update the unit test.

[TD-3144]add new query API to return sqlobj

[TD-3095]fix mem invalid read issue

[TD-3116]Fix dead lock issue

[TD-3155]fix meta retry issue

[TD-3110]<fix>: TSDB_MIN_WAL_LEVEL <- 0 to allow db wal no log

[TD-3167]<fix>: fix cannot use chinese charset in docker images

[TD-3158]<fix>: replace head_type with column_meta in http response

[TD-3154]<test>: update test case

[TD-3161]<test>add coverage on unsigned type

[TD-3127]<test>add unsigned type