This allows build chains to be traced in the common case where
an upstream job calls `laminarc queue' instead of `laminarc start'.
Incomplete job runs now have database entries, which requires
some adjustments in queries. Queued jobs can now be viewed in
the frontend and there is a corresponding status icon.
Implement a separate process, the "leader", which runs all the
scripts for a job run, instead of directly from the main laminard
process. This makes for a cleaner process tree view, where the
owning job for a given script is clear; also the leader process
acts as a subreaper to clean up any wayward descendent processes.
Resolves#78.
Improve the boundary between RpcImpl and LaminarInterface such
that the Laminar class doesn't require any types from kj/async.h.
The necessary logic moved from Laminar to RpcImpl and the notification
now happens by abstract virtual callback instead of kj::Promise.
Also remove the fairly useless 'wait' RPC call and drop the wrappers
around kj::PromiseFulfillerPair
