MEDIUM: stconn: Replace read and write timeouts by a unique I/O timeout

MEDIUM: stconn: Replace read and write timeouts by a unique I/O timeout

Read and write timeouts (.rto and .wto) are now replaced by an unique
timeout, call .ioto. Since the recent refactoring on channel's timeouts,
both use the same value, the client timeout on client side and the server
timeout on the server side. Thus, this part may be simplified. Now it
represents the I/O timeout.