Coordinator should release admitted memory per-backend rather than per-query

When SPOOL_QUERY_RESULTS is true, the coordinator backend may be long lived, even though all other backends for the query have completed. Currently, the Coordinator only releases admitted memory when the entire query has completed (include the coordinator fragment) - https://github.com/apache/impala/blob/72c9370856d7436885adbee3e8da7e7d9336df15/be/src/runtime/coordinator.cc#L562

In order to more aggressively return admitted memory, the coordinator should release memory when each backend for a query completes, rather than waiting for the entire query to complete.

Releasing memory per backend should be batched because releasing admitted memory in the admission controller requires obtaining a global lock and refreshing the internal stats of the admission controller. Batching will help mitigate any additional overhead from releasing admitted memory per backend.