Changes SettingsEnabledConnection to explicitly enable NO_BACKSLASH_ESCAPES for MySQL connections.

jOOQ assumes that MySQL databases are configured with NO_BACKSLASH_ESCAPES. This might
however not always be the case.

This commit sets NO_BACKSLASH_ESCAPES on MySQL connections. If a user wants
NO_BACKSLASH_ESCAPES but does not want to set NO_BACKSLASH_ESCAPES on each connection,
they can append ?sessionVariables=sql_mode=NO_BACKSLASH_ESCAPES to their jdbc connection string.
Exposes backslash handling as a setting.

Useful for anyone using MySQL, who decided not to enable NO_BACKSLASH_ESCAPES and
to instead manually escape strings.

The setting allows the jOOQ user to decide if they want to escape backslashes,
not escape them, or not escape them but set NO_BACKSLASH_ESCAPES on MySQL connections.
Adds a unittest for ESCAPE_BACKSLASHES.