Migrate host_task object management to a thread pool based approach

[ndgrigorian]

Inspired by comment in: AdaptiveCpp/AdaptiveCpp#1915

we move away from use of host_task, which enables compatibility with AdaptiveCpp, and fixes long-standing undefined behavior of calling sycl::free from within a host_task. The host_task decrementing the refcount would cause the USMDeleter to be called in its scope, which is undefined behavior under the SYCL spec

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2359/index.html

[coveralls]

coverage: 74.097% (+0.04%) from 74.054% — feature/drop-host-task-object-management into master