lib: add support for `Dynlink` based plugins
test: add the `dummy_plugin.ml` test

The test gets compiled by `ocp-build` and is used by `cli.ml` to start a
python-daemon. The final compilation step is for now done by `please.sh
test-env` because `ocp-build` is always more broken than expected.
lib: change the configuration of compiled plugins
test: update generation of `test-config-file.toml`
build,test: delay compilation(s) of plugin tests
build: replace `Version` with home-grown `Metadata`
lib: add support for “findlib plugins”
test: add a findlib plugin to `test-env`
lib: fix configuration-file parsing

Plugin lists had been forgotten.
build: add more testing to `do_travis`
build,test: make `test-env` more complex and portable
build: fix travis (variable instead of alias)
build: fix travis build again
test,build: add more complex testing in travis
test: add a dumb additional query to the dummy-plugin
Merge branch 'master' into dynlink
Merge pull request #2 from hammerlab/dynlink

Dynlink with Findlib support for long-running plugins