[Plasma] Store object_id only once in object table

[asfimport]

This is the first part of ARROW-2953, i.e. the duplicated storage of the object id both in the key and the value of the object hash table.

Reporter: Philipp Moritz / @pcmoritz
Assignee: Philipp Moritz / @pcmoritz

PRs and other links:

• GitHub Pull Request #2353

_{Note: This issue was originally created as ARROW-2954. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2353
#2353