Refactor reader.ReadRowsPage to use _StreamParser.

[tswast]

This will enable us to use alternative libraries and wire formats.
Previously, ReadRowsPage was closely coupled to the fastavro library.

Towards #7805 since we'll want an alternative parser and/or wire format in the future to improve performance.

[shollyman]

Looks good. Does "block" sufficiently describe the same set-of-rows avro concept in other formats like arrow, or should we call it something like binary page?

[tswast]

Arrow calls the concept "batch".

In this module, whereever I refer to "block" is really a whole ReadRowsResponse message. I've just pushed a commit that renamed it to "message" to match the GRPC terminology.