Summary:
Vector index insert tasks consume a lot of CPU and could take significant amount of time.
It is necessary to use separate thread pool for them, to avoid blocking other system activities.
Jira: DB-14167

Test Plan: PgVectorIndexTest.ManyRows/*

Reviewers: slingam, arybochkin

Reviewed By: slingam

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D40180