Pebble branch - upgrade, include fixes

[thaarok]

Introduce newer pebble version and integration improvements:

• Ensure pebble flush after SyncedPool flush (hack, clean implementation waits for core change by Egor - in development)
• Allow 3 concurrent pebble compact threads - prevents db performance degradation during big data imports (details in Pebble - allow 3 concurrent compactions for performant imports lachesis-base#43)

[thaarok]

@uprendis can this be merged to keep the pebble branch up-to-date?

[uprendis]

It's already included into #343, except for updating of pebble version. Will test performance of updated version