fix(pathfinder/sync/class_definitions): fix class definitions sync

This PR fixes a few unrelated issues in class definition sync:

finding class definition gap: there was an error in the SQL query and an off-by-one in next_missing()
verify_declared_at() did not properly go on to the next block after processing all declared classes
peer_agnostic::Client::class_definitions_stream() now handles FIN as the terminator of the response stream
peer_agnostic::Client::class_definitions_stream() did not handle class definition counts of zero (that is, blocks that did not declare classes)

eqlabs / pathfinder