[DocDB] Send GetOldTransactionStatus RPC once to each tserver

[robertsami]

Jira Link: DB-7267

Description

In the current pg_locks implementation, we send one RPC to each tserver to identify the status tablet leaders, and then one RPC to each status tablet to fetch old transactions. We should be able to consolidate these steps and reduce the total number of RPCs to one per tserver

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