Prevent fragment links corrupting when filling a gap and server returns duplicate events

[bwindels]

ignore duplicate events from synapse instead considering them an
overlapping event with the adjacent fragment

Fixes #39
Fixes #15

• if the fragment is already linked up in the direction we're filling, look up the first event id we should encounter when filling overlaps with the adjacent fragment.
• when looking for overlapping events, check if the event is the event id expected when overlapping. If not, it means we've hit Synapse returned event in backfill that was already returned during sync matrix-org/synapse#7164 and we ignore the event as it is already present in the timeline elsewhere, and find the next overlapping id in the remaining events. Repeat this until either finding the expected overlapping event id or we've gone through all the events in the backfill.
• don't add fragments to the FragmentIdComparer until the transaction has been comitted. This is another case of modify-memory-state-before-commit.