Utilize the new pool interface

[shintaro-iwasaki]

#354 introduced a new pool interface, which is more flexible and easy to implement. We should implement the following to fully utilize this new pool interface.

• New user APIs (mainly pool routines) that accept ABT_pool_context
• Clarification and revision of API descriptions
• Predefined random work-stealing pool (ABT_POOL_RANDWS)
• Predefined priority-aware pool ABT_POOL_PRIO_FIFO (or maybe ABT_POOL_PRIO_RANDWS ...?)
• More examples (ring-buffer, LIFO, fine-grained priority control, ...)

[shintaro-iwasaki]

ABT_pool functions that accept ABT_pool_context were implemented by #357. A predefined random work-stealing pool (ABT_POOL_RANDWS) was implemented by #359.

[shintaro-iwasaki]

Some users are still relying on the old customized pool API (see #366). We should create a unit test similar to https://github.com/pmodels/argobots/blob/main/test/basic/pool_user_def.c for the old custom pool API.