Add external asset ailas and definition (#278)

Add external asset ailas and definition (#278)

* Save external asset alias and definition

(cherry picked from commit 0c59d87)

* Add btm definition

(cherry picked from commit 6c8d2b5)

* rename external asset alias

* use getNativeAsset

* skip save local asset definition

* index external asset definition

* Add update-asset-alias cmd

* Use assetID as asset alias if no alias in definition

* Add update-asset-alias ErrNullAlias

* Add native_asset_definition in genesis.json

* Reverse native_asset_definition

* Unused SerializeAssetDef

* use Registry.FindByID

* use CalcExtAssetKey

* Only remove aliasCache

* Add cache newAlias to assetID

* Resolve code conflict