Async continuous batching loop

[aniketmaurya]

What does this PR do?

• Make continuous batching loop async to run prefetch in background
• Rest of the changes are the reorg for code readability.

litserve/
├── loops/
│   ├── __init__.py
│   ├── base.py                               # Contains the `_BaseLoop`  and `LitLoop` class
│   ├── simple_loops.py                 # Contains the in-built loops for single and simple batched
│   ├── streaming_loops.py           # Contains the in-built loops for single req streaming and batched streaming
│   ├── continuous_batching.py    # Async continuous batching loops

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Attention: Patch coverage is 75.35934% with 120 lines in your changes missing coverage. Please review.

Project coverage is 89%. Comparing base (fd77feb) to head (6cec895).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff          @@
##           main   #401    +/-   ##
====================================
- Coverage    91%    89%    -2%     
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  Files        25     30     +5     
  Lines      1783   1902   +119     
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+ Hits       1626   1695    +69     
- Misses      157    207    +50