fix & enhancement for udp connection handling

标点符号错误修正

fix memory leak in taos_query_a

restful commands with length between 65380 and 65536 can trigger
this issue.

1. potential data race in `taosProcessMonitorTimer`:
the issue does not exist at present because there's only one scheduler
thread, which means there's no cocurrent calls to this function for a
same `pMonitor`. but if more scheduler threads are created later,
there's a data race issue in rare case. as threads number can be
easily increased by increase the value of `taosTmrThreads`, it is very
unlikely that the developer could realize this function need to be
revised together. that's why i say it is a 'potential' issue.

this issue happens in below scenario:
a. scheduler thread1: `if (pSet)` is true and new timer is installed by
`taosTmrReset`;
b. scheduler thread1: `if (pMonitor->pSet == NULL)` is true but
`taosTmrStopA` is blocked, either by the mutex of the timer or os thread
scheduler.
c. timer thread: 200ms elapse, new call to `taosProcessMonitorTimer` is
initialized in scheduler thread2;
d. scheduler thread2: `if (pMonitor->pTimer != tmrId)` is false;
e. scheduler thread1: unblocked, stops timer and frees `pMonitor`;
f. scheduler thread2: unexpected behavior because `pMonitor` is not
valid any more.

because the result of this issue is crash or worse, i suggest to fix it
though the possibility of all the conditions are met is very very low.

2. `pthread_attr_t` related issues: per manual, an initialized
`pthread_attr_t` can be reused, should be destroyed, and the behavior of
re-init it is undefined. this issue exist in other modules also.

3. memory leaks;

4. improve failure case handling of `taosInitUdpConnection`;

5. typo

[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