Fix sync committee duty logging behavior #78
Description
Summary
If a validator is in the current sync committee and will be in the next committee as well, eth-duties will not log that it will be in the next sync committee. This is due to the fact that I use a dict which only consider the validator index as key and if a validator was already added to the dict (current sync committee) it will not be added again (next sync committee).
Possible Fix
Extend the dict to not only use the validator index as key. A combination of index and epoch seems to be appropriate. However, this change need to be made also for fetching attestation and proposing duties since the respective functions need to return the same type (see here for code entry).