Major refactor of pmdc

This merge introduces essentially a complete rewrite of the package, while preserving as much of the API as possible. The following backwards incompatible changes have been made:

• --ipc-file option removed
• --protocol dcfs option removed
• --tempdir option removed

The removed options were not used by the diskcache configuration for either datafind.ligo.org, or ARCCA (Cardiff), which suggested they were not required.

The following package features were added:

• unit tests
• python3 support
• debian builds on python3
• EL7 builds on python2 and python3
• continuous integration

/cc @thomas.downes