Add config to limit the parallelism of segment reload on Pinot Server

Segment reload itself may trigger index generation and could put large memory pressure when many segments are reloading in parallel.

Similar to realtime segment generation, we should provide a configure on Pinot Server side to limit the parallelism on segment reload.

No file or test is named. Start by examining how realtime segment generation limits parallelism and tracing the Pinot Server segment reload path; define the server-side configuration and verify that concurrent reloads are bounded to reduce memory pressure.

Written by the indexing model from the issue text.