Adlfs

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- Fixed call to isdir(), to run direct call to Azure container, instead of calling .ls on the directory
- Fixed call to isfile() to directly evaluate the file, insteading of calling ls on the directory
- Updated unit test for isdir
- Added DefaultAzureCredential as an authentication method. Activated if anon is False, and
no explicit credentials are passed. See authentication methods enabled [here](https://docs.microsoft.com/en-us/python/api/azure-identity/azure.identity.defaultazurecredential?view=azure-python).
- Updated initiate_upload() method to allow overwriting a blob if the credentials do not have delete priveleges.

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- Fixed bug in fetch_range that caused attempted reads beyond end of file
- Added check to mkdir when creating a container to confirm container_name conforms
to Azure requirements

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- Compatability with fsspec 2021.6.0
- `exists()` calls now also checks whether a directory with that name exists or not. Previously this was only checked from the cache
- Fixed bug in `find` not returning exact matches
- Added `AzureDatalakeFileSystem.rm_file` and `AzureDatalakeFileSystem.rmdir`
- Fixed bug in `filter_blobs` when target path contained a special character

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- Added the location_mode parameter to AzureBlobFileSystem object, and set default to "primary" to enable Access Control Lists and RA-GRS access. Valid values are "primary" and "secondary"

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- Now can create / destroy containers with `mkdir()`/`rmdir()` without needing to
have the privilige of listing all containers.
- When called with only container name, `exists()` now returns the proper result
(about the existence of the container) instead of a hard-coded `True`
- Removed version constraints on azure-storage-blob, pytest, and docker
- Updated conftest script to start Azurite docker container

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- fsspec version updated to 0.9.0