Only compact after executing a large number of blocks

[StephenButtolph]

Why this should be merged

After a chain has finished bootstrapping (like the P-chain) it will re-bootstrap every 10 seconds to keep up-to-tip until the full subnet has finished bootstrapping.

Calling db.Compact every 10 seconds (even with a small range of impacted values on disk) significantly reduces the performance of other DB operations.

This can cause the X-chain and C-chain syncing to be significantly slower than before this compaction was added.

How this works

Only compact the bootstrapping database if the number of blocks executed was significant.

How this was tested

• [X] Bootstrapped a mainnet node on v1.11.6 (C-chain state-sync and X-chain DAG fetching are still ongoing after >48 hours).
• [X] Bootstrapped a mainnet node with this change (C-chain state-sync and X-chain DAG fetching have finished in <24 hours).
• [X] CI

[StephenButtolph]

I'm wondering why wouldn't we compact the database inline with the block processing, i.e. within for !haltable.Halted() && iterator.Next() loop.

We tested that and it was less performant than this code. (for executing a single chain).