Manage sync-gaps

[viktorvsk]

When we start syncing with a relay for both: past and future, we get (more or less) "continuous" set of events. However, if relay is disconnected for some reason, processing starts from the same place it stopped for past events and for current time for future. Meaning, we won't get events we missed.

Syncing entire relay again is highly inefficient for this purpose. We need to have some sort of check points where we started and where we finished and sync all missing parts — gaps