[C++] Add JSON read/write support for decimals for integration tests

[asfimport]

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

Related issues:

• Integration tests for Decimal types (blocks)

_{Note: This issue was originally created as ARROW-696. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Punting off to a later release since we still need to harden the decimal memory

[asfimport]

Wes McKinney / @wesm:
PR: #1049

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1049
#1049