upgrade to datafusion 8.0.0

[jychen7]

close #4

Problem

there is a breaking change of datafusion v8

1. ExecutionPlan.execute switch from async to sync

also original implementation "should not" call bigtable.read_rows during plan phase execute. It should deferred to the return stream

Solution

split original async method execute to execute_query and use futures::stream::once and RecordBatchStreamAdapter to wrap it in plan phase execute

let stream = futures::stream::once(
    execute_query(self.clone()).map_err(|e| ArrowError::ExternalError(Box::new(e)))
).try_flatten();

let schema = self.schema();
Ok(Box::pin(RecordBatchStreamAdapter::new(schema, stream)))

To reviewers

please review by commits. Datafusion 8 requires some change on ExecutionPlan, thus I split it to another file first

[jychen7]

same as original async fn execute, separate because execute in the trait is defined as sync now

[jychen7]

this is the key to delay async execute_query to stream