[Format] Add 4-byte "stream continuation" to IPC message format to align Flatbuffers

[asfimport]

This is the JIRA corresponding to the mailing list discussion

Reporter: Wes McKinney / @wesm
Assignee: Micah Kornfield / @emkornfield

PRs and other links:

• GitHub Pull Request #4951

_{Note: This issue was originally created as ARROW-6329. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4951
#4951