[Task Manager] Perform true partial updates when claiming and releasing tasks

[mikecote]

When the mget does a bulkUpdate to change task statuses from idle to claiming or running, it uses the task store's bulkUpdate. The bulkUpdate under the hood uses the saved-object repository which recently got changed to support downward-compatible updates (#152807) by performing a get + Kibana side update before calling index. These extra requests add latency to task runtime and load to Elasticsearch that we do not need when rolling upgrades isn't a concern for claiming and releasing tasks.

Requirements

• Task store supports partial bulk updates that work directly with the Elasticsearch client. Similar to this PoC.
• Mget task claiming uses the new partial function instead of the current bulkUpdate Similar to this PoC.
• When updating recurring tasks after running, we use the new partial function to update the task document via the buffered task store

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)