update for multi-storage drivers

[jreadey]

This PR enables HSDS to support different storage back ends (posix, s3, azure blob) in the same running instance. If the bucket query parameter is pre-appended with a storage specifier, the corresponding driver will be used for reads and writes. Currently supported prefixes are:

• "s3://" - aws s3
• "file://" - posix
• "https://account_name>.blob.windows.net/" - azure blob storage

If no prefix is used, the default driver will be used. Where default is the first one meeting the given precondition from this list:

• "file://" - if root_dir config is set
• "s3://" - if aws_s3_gateway is set
• "https....net/" - if azure_connection_string is set
• "file://" - if none of the above apply