[JVPP-2] Take shm prefix as paramenter of VppJNIConnection constructor instead of SHM region name

[vvalderrv]

Now once needs to give full name, e.g. "/prefix-vpe-api"

 

The change is required for consistency with VPP config:

https://wiki.fd.io/view/VPP/Command-line_Arguments#.22api-segment.22_parameters