Convert source verification to use xiasecurity lib

The source verification was using ad-hoc code to build, serialize and
verify messages from a new host. This code now uses the same xiasecurity
library used by the service migration code.
This is a large-code change and can break things, so report any
problems.
Merge branch 'intrinsic_security' of github.com:XIA-Project/xia-core into intrinsic_security