Implement worker coroutines for asynchronous background processing

This implements worker coroutines in Lua to perform non-blocking I/O and do many things concurrently.
For example a file watcher can be now implemented as:

```
  local watcher = notify.opendir('/etc')
  watcher:add('hosts')

  -- Watch changes to /etc/hosts
  worker.coroutine(function ()
    for flags, name in watcher:changes() do
      for flag in notify.flags(flags) do
        print(name, notify[flag])
      end
    end
  end)
```

In order to make this work, the runtime uses the cqueues library which
can run coroutines concurrently, and return a file descriptor to poll on
if it's blocked. The worker takes that file descriptor and calls
`event.socket(pollfd, resume_callback)` so that libuv can wake up
the worker when its ready again.

The cqueues library is still optional, but if it's not present following stuff
won't work:

* worker.coroutine()
* worker.sleep()

1 files changed, 1 insertions, 2 deletions

diff --git a/modules/daf/daf.lua b/modules/daf/daf.lua
index 2ddbe032..28b6342b 100644
--- a/modules/daf/daf.lua
+++ b/modules/daf/daf.lua
@@ -296,7 +296,6 @@ end
 
 -- @function Publish DAF statistics
 local function publish(_, ws)
-	local cqueues = require('cqueues')
 	local ok, last = true, nil
 	while ok do
 		-- Check if we have new rule matches
@@ -318,7 +317,7 @@ local function publish(_, ws)
 		else
 			ok = ws:send_ping()
 		end
-		cqueues.sleep(1)
+		worker.sleep(1)
 	end
 end