[Python] Add classes / functions to enable stream message components to be handled outside of the stream reader class

[asfimport]

This will be helpful in messaging / RPC scenarios where the message components are sent in multiple streams (e.g. schema negotation first, then record batches later)

• Class for holding a single serialized message, able to indicate what kind of message it is
• Read schema from list of one or more messages (if there are dictionaries, there will be multiple messages, otherwise only one)
• Read record batch from a message given the schema

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [Java] Add generalized stream writer and reader interfaces that are decoupled from IO / message framing (relates to)

_{Note: This issue was originally created as ARROW-1214. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #839

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 839
#839