process.c: Introduce work queues

process.c: Introduce work queues

Work queues are a way to execute function calls asynchronously. Each
work is placed on a fifo queue where worker threads pick them up one
by one.

Priority levels are also supported, right now two levels are
available. All high priority works are executed before workers pick up
low priority works.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>