Enh: Use better serie naming convention.

Use "encode_serie_name" function.

This will :

- allow to correctly get them back when reading influxdb.
- not have conflicts between hosts/services names (though.. this depends on the select query we'll use).
- "simplify".
- use a lock around buffer attribute. 2 threads could access it simulatenously.

Added some tests.

tbc.. (TODO).