feat: optimize staking EndBlock/BeginBlock queue iterators

[thomas-nguy]

Summary

Reduces staking EndBlock (and BeginBlock historical info) cost when chain and queue data grow by avoiding full-prefix scans on time/height-ordered queues. Iterators now start from a tracked queue head (or, for HistoricalInfo, a min height) and can early-exit when there are no mature entries.

Problem

Profiling showed bottlenecks in:

• ValidatorQueueIterator
• UBDQueueIterator
• RedelegationQueueIterator

Each was iterating from the prefix start (e.g. time 0) up to the current block time/height every block. As the chain and queues grew, that range and the cost of creating/using the iterator increased even when only a few entries matured per block.

Solution

1. Queue head tracking (Validator, UBD, Redelegation)

• Persist the minimum (earliest) completion key still in each queue.

• Iteration: Start the iterator from this head key instead of the prefix start (fallback to prefix start if head is missing).

• Early exit: If the stored head is after current block time/height, skip creating the iterator and return.

• Head updates:
  On enqueue (e.g. SetUBDQueueTimeSlice, SetRedelegationQueueTimeSlice, SetUnbondingValidatorsQueue): set head when the new key is earlier than the current head (or head is unset).

  On dequeue (UBD and Redelegation only): after removing mature timeslices, set head to the next remaining key, or clear head if the queue is empty. 
  

• Validator queue: head is updated only on enqueue; if the head key is later deleted, the next block’s iterator still starts at that key and the store seeks to the next existing key.

2. HistoricalInfo (BeginBlock)

• Persist the minimum stored height for historical info.
• IterateHistoricalInfo: Start from GetHistoricalInfoKey(minHeight) when the min-height key is set.
• TrackHistoricalInfo: After pruning, set the min-height key to blockHeight - entryNum + 1.
• SetHistoricalInfo: When the min-height key is missing (e.g. genesis), set it to the current height so iteration is bounded from the first use.

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[randy-cro]

I remember I did something similar to this, ie trying to reduce the range of iteration, before trying to create an in memory cache. However, the duration taken to create the iterator still causes significant lag in the endblocker, staking endblocker is still taking around 700ms

[randy-cro]

dont think historical info is exported or imported for genesis