core: Parameterize message validator in Session

Keep 'Config' before 9b7ee5f801108bed44caa0cc72fab5f521e86309 was merged and
pass an instance of 'Validator<Message>' to 'Session'.

Rename 'MessageValidator' to 'DefaultMessageValidator' and use instance methods
instead. This allows customized validation logic through overriding the default
implementations. In addition, through customization, this approach allows, for
instance, more filtering of accepted SenderCompID and TargetCompID upon
authentication.

Allow only point-to-point messaging by disallowing OnBehalfOfCompID(115) and
DeliverToCompID(128) by default. Drop 'CompIdValidator' as no longer used.

Signed-off-by: Karim Vuorisara <karim.vuorisara@valotrading.com>