1. Simplify implementation of taosHashCondTraverse
2. Complete function header comments for a few more functions

* Testing

Run through the sample mysql:
taos> create database db;
Query OK, 0 of 0 row(s) in database (0.002475s)

taos> use db;
Query OK, 0 of 0 row(s) in database (0.000252s)

taos> create table t (ts timestamp, a int);
Query OK, 0 of 0 row(s) in database (0.003588s)

taos> insert into t values ('2019-07-15 00:00:00', 1);
Query OK, 1 of 1 row(s) in database (0.001268s)

taos> insert into t values ('2019-07-15 01:00:00', 2);
Query OK, 1 of 1 row(s) in database (0.000265s)

taos> select * from t;
           ts            |      a      |
========================================
 2019-07-15 00:00:00.000 |           1 |
 2019-07-15 01:00:00.000 |           2 |
Query OK, 2 row(s) in set (0.000822s)

taos> drop database db;
Query OK, 0 of 0 row(s) in database (0.001711s)