SQL Query improvements

[oqq]

This PR provides changes on queries for hasStream requests on MySQL and Postgres.

On MySQL, this query could be done only with index file and without fetching real data from db-table.
Since the stream_name is not relevant there is no need to fetch it, which results in a performance improvement.

Not sure about Postgres, but should be the same how on MySQL.

Note that fetchColumn for MySQL would return a string, but an integer on Postgres.

[coveralls]

Coverage decreased (-0.0008%) to 99.216% when pulling d78f4ab on oqq:improvements/sql_queries into 2d57e0c on prooph:projections_update.