Buffer for updateContext "bulk" processing

[fgalan]

Current Orion version (0.23.0) processes updateContext containing N elements in sequential mode, doing N query/insert operations, even when the N refer to the same entity/attributes. This is a suboptimal behaviour.

It would be better to keep a buffer while updateContext is being processed, doing the modifications (and any futher brokering logic, i.e. notification on changed values) in the buffer. The buffer will be consolidated in the DB once updateContext processing is finalized.

[fgalan]

In some sense, it is related with #881

[fgalan]

This MongoDB operation could be used http://mongoc.org/libmongoc/current/mongoc_bulk_operation_update.html, accumulating all the documents to be updated.

This bulk could also include the update to upgrade fiware correlators in DB, as explained in #3893 (comment)