Add dynamic support to omf_web_server

Seems that both :limit and :check_interval should be supplied to
OmlSqlRow, not OmlSqlSource.

Originally I thought :check_interval would trigger periodical checking
unconditionally, but seems that it only works when data fetched each
time is larger than the default 1000 limit.

Therefore for small data stream (e.g. 1 row per sec added), we need to
set check_interval to much higher (e.g. 5 sec), and :limit to much lower
(e.g. 2 rows).