[Java] Avro to Arrow converter should reuse the same VectorSchemaRoot

[asfimport]

When originally reviewing the code I suggested a new VectorSchemaRoot on each call to the iterator.  After further discussions on the mailing list, it seems that this is an anit-pattern for working with VectorSchemaRoot, we should update the code to update a single VectorSchemaRoot.

 

After this change it should be easier to use Avro converter with other components of the library (i.e. filewriter) which also make use of a single VectorSchemaRoot.

Reporter: Micah Kornfield / @emkornfield

_{Note: This issue was originally created as ARROW-9613. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.