Limit memory usage

[forceedge01]

We're hitting memory limit issues on mac os x behat 2.5 and behat 3.0 when using the sql extension with over 6000 steps and PHP memory limit of 384 mb.

Ideas on how to solve this problem from the sql extensions perspective.

Reduce the memory footprint by:

• Make the SQLHistory object optional. Don't record history if not provided.
• Documenting that the local keystore can be replaced by a solution that stores keys externally such as Redis keystore.

This needs to be applied on both 2.5.x and 3.x versions.

[forceedge01]

Also consider removing duplicate entries in the keystore, at the moment we get this stored:

[db.table.column] => value
[db.table_column] => value

[forceedge01]

Also need this in place after initialising PDO connection:

$this->connection->setAttribute(\PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false);