Implement version checks between CVMFS gateway and cvmfs_receiver

There should be version checks between the CVMFS gateway component and the cvmfs_receiver executable. This should be done in 2 complementary ways:

1. The cvmfs-gateway package should depend on cvmfs-server and enforce a minimum version.
2. The cvmfs-gateway application, when spawning the cvmfs_receiver exectuable, should perform a handshake exchange, to ensure that the version of the receiver is compatible. This approach also works when the software is installed from source.