WL#8320: Lower default slave_net_timeout in 5.7

Affects: Parser-5.7   —   Status: Complete

This work changes the default of slave_net_timeout to "60" (was: 3600). 

This avoids long replication delays due to periodic network failures. 

See also BUG#74607: SLAVE IO_THREAD MAY GET STUCK WHEN USING GTID AND LOW
SLAVE_NET_TIMEOUTS