Add missing synchronization to getBestCapacity:
- This didn't cause any bugs as its only caller is synchronized on the WebOfTrust anyway
Make shouldFetchIdentity() as it is only used internally and also shouldn't be needed in clients:
- They should only look at the score of an identity. Capacity is only used as a fetch decision in WOT because we need to import trustees of identities which have capacity
JavaDoc synchronization requirements of shouldFetchIdentity():
- They are already met by all callers.