[C++] Add support for reading Flight streams with dictionaries

[asfimport]

Some work is needed to handle schemas sent separately from their dictionaries, i.e. ARROW-3144. I'm going to punt on implementing support for this in the initial C++ Flight client

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] Move implementations in arrow/ipc/test-common.h into libarrow_testing (supercedes)
• [C++] Add an incomplete dictionary type (depends upon)

PRs and other links:

• GitHub Pull Request #4113

_{Note: This issue was originally created as ARROW-3200. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
cc @pitrou, for thoughts. My opinion is that we should make arrow::DictionaryType into a virtual interface and define a MutableDictionary subclass to permit the Flight client or server to add the dictionary when the message comes across the wire. This will require some refactoring in the IPC code to permit schemas to be decoded while leaving the dictionaries temporarily null

[asfimport]

Antoine Pitrou / @pitrou:
Some kind of "IncompleteDictionary" would probably be useful for ARROW-3408 as well, to denote a dictionary whose values are still (partially?) unknown.

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 4113
#4113