dbwrap: add enum dbwrap_req_state

dbwrap: add enum dbwrap_req_state

This will be used by async dwrap_parse_send() as an out argument, giving
the caller an indication about the state of the request.

This is can be useful for the caller if it is a sync function and sends
multiple async dbwrap requests. As it's a sync function it won't return
to the main tevent event loop and so the async dbwrap recv function are
not called.

As a result the function may deadlock: our receive queue may already be
full with results from a peer, the peer might be blocked in his send
queue (because we're not receiving), the peer therefor doesn't read from
his receive queue so our send queue will block as well.

To inform the caller of this situation "send queue full" we return this
state information to the caller of the dbwrap send function.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>