Refactoring: renamed EventPosition to TFPos in order to allow future merge
with core TFPos
Refactoring: merged TFPos and EventPosition structs
Added: test fixtures with existing events - auto building log position
Added: basic read by indexes feed-reader test
Fixed: TestFixtureWithExistingEvents commit/prepare position handling +
MultiStreamReader maxN handling
Added: test joining streams (already committed reordered index entries)