Displaying API numbers originating from mongodb as unsigned

[fboucquez]

currently, unsigned data fetched from mongodb is displayed as signed since mongodb does not support unsigned data

consider treating every number in the project as unsigned unless specified since the vast majority are unsigned, or add and parse every number in the schemas even if basic type (which wouldn't be a bad idea anyway from a documentation/explicitness point of view)

symbol/catapult-rest#323
symbol/catapult-rest#144