Add configuration per engine instance and per session

[ajnavarro]

We need a way to set configurations at the engine (or BBDD) and session levels. This configuration should be of two types, one specific to configure parameters on go-mysql-server (log level, number of rows to return per batch on a connection) or specific depending on the database implementation (on the gitBase case, the max size of files to return in blob table)

[erizocosmico]

Should that be in a file, environment variables, SET commands, ...?

[ajnavarro]

Closing as outdated. We have session and global variables implemented now.