Issue #397 fix

snapshot dump and load extended by generic_index::_next_id to prevent update_proposal failures on nodes configured using snapshot. Originally, even correctly, next_id in the generic_index structure has been set to the value following the last object ID held in given index. The difference between last object ID and next_id would occur when some objects have been removed from the index. Usually id values (even unique) are neglible against consensus rules - except create_proposal_operation/update_proposal_operation. Maybe it is worth to consider replacing this operation with new explicit_create_proposal_operation where proposal id should be passed as part of operation.