Ureserv Inc., the online reservation system quietly taking on OpenTable, said this week its system had processed its 10 millionth restaurant reservation and expanded into seven countries. The Belmont-based company — co-founded by engineer and sales executive Van Garrett and Boston restaurateur Jeffrey Gates, partner and owner of The Aquitaine Group— said that it has also expanded to seven countries. Garrett said there are now 450 restaurants using Ureserv's system. Notable clients include The…