Btrfs: don't use an async starter for most of our workers

We only need an async starter if we can't make a GFP_NOFS allocation in our
current path.  This is the case for the endio stuff since it happens in IRQ
context, but things like the caching thread workers and the delalloc flushers we
can easily make this allocation and start threads right away.  Also change the
worker count for the caching thread pool.  Traditionally we limited this to 2
since we took read locks while caching, but nowadays we do this lockless so
there's no reason to limit the number of caching threads.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>