lib: make LSF backend work with older versions

This fixes #15.
test: fix LSF-job (integration)
lib: implement “restarting” in client-server mode
lib: expose function `Engine.database`
lib: start keeping track of “measurements”

For now we keep track of HTTP requests seen by the server.
The measurements are flushed to the database when the engine is
“released”, or when the server is asked to through the command-pipe.