Fixes for Clang 15 warnings

[HennerM]

Related to triton-inference-server/server#6768,
was trying to compile latest main with Clang 15 and got compiler warnings that showed up as error with default CMake settings

• unused private member backend_thread_exit_ in TritonModelInstance (Wunused-private-field)
• missing virtual destructors [-Werror,-Wdelete-non-abstract-non-virtual-dtor]
• Missing explicit copy constructor for SequenceId, resulted in error: definition of implicit copy constructor for 'SequenceId' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
• missing copy assignment operator leading to same error in BatcherSequenceSlot
• multiple unused capture variables: -Wunused-lambda-capture
• unused result in ditr->second.emplace(name, count).first

[HennerM]

@dyastremsky can you have a look at this?

[the-david-oy]

Thanks for submitting these! Sorry we did not see them until now. PRs in the non-server repo sometimes get missed. I created an internal ticket (DLIS-6450) for someone to take a look at this.

Can you submit the Contributor License Agreement (CLA)? We will need that before reviewing this pull request.