Improvements to queue manager #265

This is a blanket issue covering various improvements to the dpctl queue manager:

• All types of queues should be supported by dpctl including the "host" queue. (Queues creation should be supported for all available SYCL devices #311)
• Allow queue creation to support properties and async_handler. (Allow Sycl queue creation with an async_handler and supported SYCL properties #312)
• dpCtl should support SYCL's filter selector. (Specifying backend for device_context #100)
• A SYCL context per device should be cached and the cached context should be used for queue creation. (dpctl shuold use a primary or default context per root device #313)
• The queue manager should support Python asyncio and be async task-safe. (Thread-safety of device_context #11)
  • The Python queue manager object should be a contextvar.
  • The C API functions should be made reentrant and not use global objects.
• Change how the default/global queue is set and reset in dpctl. (Functions for selecting current device #247)
• Allow creating a device_context from an existing queue object.
• Allow creating a queue from an existing device object (Allow creating a SyclQueue from an exisiting SyclDevice (possibly sub-device) or SyclContext #266)
• A helper function to print out a list of devices that can be used must be included in the queue manager. (SyclContext must have method to give list of devices #146)
• Add a way to enumerate list of available devices (Functions for enumerating devices #248)
• Add a way to enumerate list of available platforms.
• Obey all DPC++ runtime settings when setting defaults for dpctl (The default filter selector should be based on SYCL BE settings #267, dpctl should better query SYCL run-times for actually available backends.  #139, Environment variables for configuring of default queue #131)
• Add GetNumDevices helper function.