Attribute ID metadata support in updateContext and queryContext

Unit tests and harness tests passed with 100% success.

Attribute ID metadata is not finished: it needs to be also supported in notifyContext send due to subscribeContext operations. This is not included in this PR due to the following reasons:

1. To get early feedback, as result of the code review comments
2. Not to block @kzangeli on implementing the GET/POST/DELETE operations /ngsi10/contextEntities/{EntityID}/attributes/{attributeName}/{valueID} convenience operations