Fix NULL pointer access with asyncio when Squid starts up over the disk

Fix NULL pointer access with asyncio when Squid starts up over the disk
space limit.

We used to call squidaio_init() from DiskThreadsDiskFile::operator new.
But this only gets called when opening a file.  If the first transaction
is to unlink a file, squidaio will be uninitialized.

I didn't see any other good places to call squidaio_init().  For now
I am calling it from within the squidaio module itself at the
start of each open, unlink/truncate, and stat call.