Remove multiprocessing.pool.ThreadPool

[alcarney]

Description (e.g. "Related to ...", etc.)

This PR makes what should be a small breaking change to simplify the base JsonRPCServer slightly by removing the use of multiprocessing.pool.ThreadPool.

According to the docs, the only difference between the ThreadPool and ThreadPoolExecutor is the API. They even recommend the use of the ThreadPoolExecutor over the ThreadPool as the Future implementation used is compatible with other standard library modules - such as asyncio.

As a result

• JsonRPCServer.thread_pool now returns a ThreadPoolExecutor
• JsonRPCServer.thread_pool_executor has been removed
• Message handling in JsonRPCProtocol has been tweaked to make use of the ThreadPoolExecutor for threaded handlers.

Unless a server actively used the thread_pool or thread_pool_executor attributes they are unlikely to even notice the change. Handlers registered with the @server.thread() decorator should continue to work unmodified.

---

While I was making changes to JsonRPCProtocol I've also tweaked the logs generated when the server receives an unknown request method - fixes #443

Before

pygls.protocol.json_rpc: error: Failed to handle request 2 textDocument/documentSymbol DocumentSymbolParams(text_document=TextDocumentIdentifier(uri='file://path/to/project/debian/changelog'), work_done_token=None, partial_result_token=None)
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pygls/protocol/json_rpc.py", line 218, in _get_handler
    return self.fm.builtin_features[feature_name]
           ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
KeyError: 'textDocument/documentSymbol'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pygls/protocol/json_rpc.py", line 221, in _get_handler
    return self.fm.features[feature_name]
           ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
KeyError: 'textDocument/documentSymbol'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pygls/protocol/json_rpc.py", line 260, in _handle_request
    handler = self._get_handler(method_name)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/pygls/protocol/json_rpc.py", line 223, in _get_handler
    raise JsonRpcMethodNotFound.of(feature_name)
pygls.exceptions.JsonRpcMethodNotFound: Method Not Found: textDocument/documentSymbol
pygls.protocol.json_rpc: info: Sending data: {"error": {"code": -32601, "message": "Method Not Found: textDocument/documentSymbol"}, "jsonrpc": "2.0", "id": 2}

After

pygls.protocol.json_rpc: warning: Failed to handle request 2, unknown method 'textDocument/documentSymbol'
pygls.protocol.json_rpc: info: Sending data: {"error": {"code": -32601, "message": "Method Not Found: textDocument/documentSymbol"}, "jsonrpc": "2.0", "id": 2}

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

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[tombh]

As ever a wonderful PR ❤️ Clean code, updated docs, good tests. Thank you.