[WIP] Use inferred return type for all accumulator functions.

[lowka]

Thank you for submitting the pull request.

To streamline the review process of the patch and ensure better code quality
we ask both an author and a reviewer to verify the following:

The Review Checklist

• Formal criteria: TC status, codestyle, mandatory documentation. Also make sure to complete the following:
  - There is a single JIRA ticket related to the pull request.
  - The web-link to the pull request is attached to the JIRA ticket.
  - The JIRA ticket has the Patch Available state.
  - The description of the JIRA ticket explains WHAT was made, WHY and HOW.
  - The pull request title is treated as the final commit message. The following pattern must be used: IGNITE-XXXX Change summary where XXXX - number of JIRA issue.
• Design: new code conforms with the design principles of the components it is added to.
• Patch quality: patch cannot be split into smaller pieces, its size must be reasonable.
• Code quality: code is clean and readable, necessary developer documentation is added if needed.
• Tests code quality: test set covers positive/negative scenarios, happy/edge cases. Tests are effective in terms of execution time and resources.

Notes

• Apache Ignite Coding Guidelines