Allows multiple requests per server per request ID

[egalpin]

Relates to https://github.com/apache/pinot/issues/10712

This PR proposes to remove the concept of separate "servers" for OFFLINE and REALTIME query handling. Instead, queries are uniquely identified based on the physical table that they target in the actual query (myTable_OFFLINE or myTable_REALTIME). The hashcode of the Thrift PinotQuery object is unique per-server, per-physical-table.

This change helps pave the way for Logical Table support by allowing a single broker request to more easily "fanout" into arbitrarily many requests issued to each required server.

There may be a few rough edges here and there, but I'd like to open this for feedback on the concept and current implementation.