allow user to set starting compress threshold value in odbc driver

[Daniel Lu]

Currently, driver compression is turned on by the DSN setting "Compression=?", but still, if the sending buffer size is too small, we do not compress it. because in this case the time would be taken by compress+extract code, very possible longer than actually transmit time of the small buffer. so we have a hard code threshold value for this in driver and server. but we know what is the best value for this depend on actual H/W and network delay. so hard code is not a good option. we need to externalize it to outside user. for driver, we can put it in DSN setting, but for server side, a new property is required for DCS server, we need consider about it.