Storyline: #335

Immediately following START SLAVE command, gh-ost sleeps for 500ms to allow the replica the time to connect to its master.
Otherwise, if SHOW SLAVE STATUS is immediately executed after START SLAVE, the Slave_IO_thread can still be in Connecting state, which is interpreted as "oh replication isn't running".

• contributed code is using same conventions as original code
• code is formatted via gofmt (please avoid goimports)
• code is built via ./build.sh
• code is tested via ./test.sh