This PR adds appropriate pinnings for `gtest` and `gmock` in `libucxx-tests`.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Peter Andreas Entschev (https://github.com/pentschev)
  - Ray Douglass (https://github.com/raydouglass)

URL: rapidsai#66

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This PR introduces various fixes:

* Fix destruction of `ucxx::RequestTagMulti`, which after moving to inheriting from `ucxx::Request`, using `setStatus()` was missing, thus causing the `ucxx::RequestTagMulti` to never cleanup its inflight request status. Calling `setStatus()` instead of writing the `_status` attribute directly resolves this.
* The status of `ucxx::RequestTagMulti` was until now always `UCS_INPROGRESS` or `UCS_OK` after completing. This is not right but there is no good way to combine the statuses of all underlying requests. Therefore we now set the status of the first failing request as the final status instead of `UCS_OK` if at least one of the requests failed. The user can still check each underlying's request status if granular information is required.
* Since `ucxx::RequestTagMulti` now inherits from `ucxx::Request`, the latter's trace logging for creation/destruction are sufficient, thus removing the redundant trace logs is appropriate.
* Add lifetime trace logs for `ucxx::BufferRequest`.
* Execute user-defined callback within the scope fo `ucxx::Request::setStatus()` to prevent accidental reordering mistakes, as the callback needs to always execute after the status is set, because the callback itself might make use of that information. Additionally it will prevent missing execution of the callback should `setStatus()` be called elsewhere in the future.
* Return `std::shared_ptr<Buffer> in allocateBuffer()` as returning raw pointers may cause leaks, and can be problematic specifically in `ucxx::RequestTagMulti` as the `Buffer*` will not be released unless the user takes ownership and releases it appropriately.
* Introduce mutex in `ucxx::Request` because of potential concurrency issues when running with the progress thread, which a mutex can resolve in `ucxx::Request`. A common case is when `ucxx::Endpoint` is registering an inflight request and `setStatus()` has just begun running, in which case the inflight request will be removed (a no-op as it hasn't been previously registered) and then actually registered, which will cause the `ucxx::Request` to never cleanup.

Authors:
  - Peter Andreas Entschev (https://github.com/pentschev)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: rapidsai#72

…sai#65)

Implement and use generic callbacks to create endpoints, close endpoints and listeners, and probe tags. By doing this we ensure those don't need to take the UCS spinlock or progress the worker during the application thread, which can be sources of deadlocks.

Authors:
  - Peter Andreas Entschev (https://github.com/pentschev)
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: rapidsai#65

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