[taosdemo] repeat insertation if failed, report failure when inserting 5 times

Issue 349

Update taosSqlCgo.go

这个dbname都没有传进行，在连接池的情况下，每个连接都在选 use db.

当连接池切换连接时，就会出现 invalid DB。
```
8/16 12:44:46 dsn.go:59: input dsn: root:taosdata@/tcp(127.0.0.1)/sensors_test
TAOS DRIVER 2019/08/16 12:44:46 dsn.go:130: cfg info: &{user:root passwd:taosdata net:/tcp addr:127.0.0.1 port:0 dbName:sensors_test params:map[] loc:0xed6be0 columnsWithAlias:false interpolateParams:true parseTime:true}
TAOS DRIVER 2019/08/16 12:44:46 taosSqlCgo.go:52: taosQuery() input sql:INSERT INTO d_BDCF682B78F44364AA551B0F5E6BD1F6_acc USING acc_data TAGS ('BDCF682B78F44364AA551B0F5E6BD1F6','20190816-
TAOS DRIVER 2019/08/16 12:44:46 taosSqlCgo.go:61: taos_query() failed: invalid DB
```

enhance robustness of scheduler

fix dead code

upload code for cassandra/tdengine test

1. check all memory allocation failure in `taosInitScheduler`;
2. make `pSched->numOfThreads` the actual number of created threads
to avoid crash in `taosCleanUpScheduler` when `pSched->qthread` is NULL
and other issues;
3. check interruption of `sem_wait` in `taosScheduleTask`,
and use `pTrace` instead of `pError` when `sem_wait` was interrupted;
4. cancel all threads before join them to enable cocurrent cancellation;
5. remove unused global variable.

alter displaying information for default value of some options in taosdemo

change direcotry from taosbenchmark to taosdemo in CMakeList.txt

Return records with seperated fields in Lua connector.

fix memory leak in os/linux/src/os.c::taosInitTimer #304