Use contexts in tests
Modify to use lthread and ltchan, instead of libtask.

This is because libtask does not play well with pthreads,
while lthread actually polls fds on different threads to
prevent the lockup we've been experiencing.
Attempt to utilise the cleanup handler. Fail to.

For some reason, it's generating compilation errors on
uncommenting it; maybe check is doing something?
Commit rest of tests. [failing: round_robin_async]
Remove libtask completely; fix some minor bugs.
Fix round robin tests.
Merge pull request #1 from robxu9/dev_lthread

Move from libtask to lthread & ltchan.