[C++] Handle range of Decimal128 values that require 39 digits to be displayed

[asfimport]

2 ** 127 - 1 and -2 ** 127 both require a 39th digit to be displayed as a decimal string.

This means we do not have the ability to display decimal values above 10 ** 38 - 1 even though we have the ability to store them.

For reference, Impala returns NULL when trying to cast 2 ** 127 - 1 to DECIMAL(38, 0).

A few next steps are in order:

1. Explicitly test this behavior
2. Avoid crashing or displaying garbage when we cannot display a number that can be stored
3. Make a decision about whether we want to eventually support range of values from 10 ** 38 to 2 ** 127 - 1 positive and -2 ** 127 to -10 ** 38 negative. This would require a larger integer type to hold the value just before being converted to a string.

Reporter: Phillip Cloud / @cpcloud
Assignee: Phillip Cloud / @cpcloud

PRs and other links:

• GitHub Pull Request #1282

_{Note: This issue was originally created as ARROW-1749. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1282
#1282