In memory geo-subscription checking

[fgalan]

Geo-subscriptions functionality was implemented in #1678. However, the implementation is sub-optimal. As described in the documentation (perf_tunning.md):

Current support of georel, geometry and coords expression fields in NGSIv2 subscriptions (aka geo-subscriptions) relies in MongoDB geo-query capabilities. While all the other conditions associated to subscriptions (e.g. query filter, etc.) are evaluated on a memory image of the updated entity, the one related with the georel, geometry and coords of a given subscription needs a query on the DB.

However, note that the impact on performance shouldn't be too heavy (the operation invoked in MongoDB is count() which is relatively light).

More specifically, processSubscriptions() check 3 should be modified, in order to avoid using collectionCount() at the end.

Some research on GeoJSON libraries in C/C++ needs to be done to solve this issue. It doesn't seem feasible to implement out own GeoJSON evalutation engine.