versioning shared libraries

[carns]

Not urgent since Margo installs are usually partitioned off into spack package directories, but we should probably consider following standard library versioning practice.

Right now the shared library is installed as libmargo.so.0.0.0.

[carns]

This probably applies to several other Mochi libraries as well. We need to consider the semantic conventions for the major/minor/release numbers in the library context and adopt a consistent versioning strategy.