context, status and hooks: progressPct and CurrentLag

[shlomi-noach]

A few changes for different objectives but related in essence:

• Compute and store progressPct in migration context
• Output replication lag in short status
• Support GH_OST_INSPECTED_LAG and GH_OST_PROGRESS in hooks.

Related: #784

[shlomi-noach]

Copy: 72500/375190 19.3%; Applied: 0; Backlog: 0/1000; Time: 7s(total), 4s(copy); streamer: mysql-bin.015777:27167004; Lag: 1, State: migrating; ETA: 16s
Copy: 97500/375190 26.0%; Applied: 1; Backlog: 0/1000; Time: 8s(total), 5s(copy); streamer: mysql-bin.015777:35686181; Lag: 0, State: migrating; ETA: 14s
Copy: 122500/375190 32.7%; Applied: 2; Backlog: 0/1000; Time: 9s(total), 6s(copy); streamer: mysql-bin.015777:43903526; Lag: 0, State: migrating; ETA: 12s
Copy: 147500/375190 39.3%; Applied: 3; Backlog: 0/1000; Time: 10s(total), 7s(copy); streamer: mysql-bin.015777:52202838; Lag: 1, State: migrating; ETA: 10s
Copy: 172500/375190 46.0%; Applied: 3; Backlog: 0/1000; Time: 11s(total), 8s(copy); streamer: mysql-bin.015777:60461451; Lag: 0, State: migrating; ETA: 9s
Copy: 195000/375190 52.0%; Applied: 3; Backlog: 0/1000; Time: 12s(total), 9s(copy); streamer: mysql-bin.015777:68007845; Lag: 0, State: migrating; ETA: 8s
Copy: 222500/375190 59.3%; Applied: 3; Backlog: 0/1000; Time: 13s(total), 10s(copy); streamer: mysql-bin.015777:77016267; Lag: 1, State: migrating; ETA: 6s
Copy: 247500/375190 66.0%; Applied: 3; Backlog: 0/1000; Time: 14s(total), 11s(copy); streamer: mysql-bin.015777:85187626; Lag: 0, State: migrating; ETA: 5s
Copy: 272500/375190 72.6%; Applied: 3; Backlog: 0/1000; Time: 15s(total), 12s(copy); streamer: mysql-bin.015777:92846416; Lag: 0, State: migrating; ETA: 4s
Copy: 295000/375190 78.6%; Applied: 3; Backlog: 0/1000; Time: 16s(total), 13s(copy); streamer: mysql-bin.015777:100874582; Lag: 1, State: migrating; ETA: 3s
Copy: 320000/375190 85.3%; Applied: 3; Backlog: 0/1000; Time: 17s(total), 14s(copy); streamer: mysql-bin.015777:109023578; Lag: 1, State: migrating; ETA: 2s
Copy: 350000/375190 93.3%; Applied: 3; Backlog: 0/1000; Time: 18s(total), 15s(copy); streamer: mysql-bin.015777:118558878; Lag: 0, State: migrating; ETA: 1s