Add pvname -> real_name cache

[ralphlange]

Original LaunchPad Bug #1404307 reported by Ralph Lange on 2014-12-19:

The Gateway checks a new pvname against the regexps twice: Once in pvExistTest() and then again in createPV().

Maybe there should be some kind of cache that keeps the information and prevents the gateway from matching and creating the real_name twice.

Created from Mantis issue 10