Functionality for running a multi-build api server.  This is very similar to the
single-build api server, but it allows to register multiple builds, each with
their own token.  The token must be specified in the request, and it allows the
request handlers to determine the build associated with the request.

Agent event handlers.  These process `build/queued` events and start the build
scripts in child container processes.

Main entry point for the build agent.  This is a process that picks up `build/queued`
events and runs them as container child processes.  There is a single build api server,
hosted by the agent process that serves all builds.  This allows to run the builds on
one or more virtual machines, which is faster than provisioning containers for each
build separately, albeit somewhat less efficient if there are few builds.  Because
build scripts are mostly waiting for containers to finish, we can stack many parallel
builds on one VM.

It may be possible to suspend the build VMs after a certain inactivity timeout, to
conserve computing capacity.

Sets up the runtime that is used by a build agent.