WL#1907: Add client timeouts for read/write

Now there is no way for a client to timeout on a read/write to the server.
In the case of the server going down, the client may hang up to 10 minutes 
(Linux keep-alive is not configurable per connection and default is quite high)
See http://bugs.mysql.com/bug.php?id=4143

What to do:

- If the user sets readtimeout/writetimeout with mysql_options(), invoke in 
net.c code that will abort if the timeout is reached.
- If one doesn't use timeouts, we should use simple read/writes to get maximum 
speed.

One should be able to do it similar to the way we handle connect() timeouts in 
libmysql.c