Add Block Number Cache to Staged Stream Sync

[GheisMohammadi]

During staged stream sync, the node repeatedly queries every connected peer for their current block height — both in identifySyncedStreams (to filter peers that are synced past the target) and in estimateCurrentNumber (to determine the sync target).

For long-range syncs, these queries are almost entirely redundant. If a peer reports block 1,200,000 and our current target is 500,000, there is no reason to ask that peer again until we actually catch up to 1,200,000.

This PR introduces a per-stream block number cache that eliminates those repeated round-trips, significantly reducing network load on both the syncing node and its peers.

The BlockNumberCache stores each peer's last-reported block height keyed by stream ID.

• On a cache hit — when the cached value already meets or exceeds the current target — the cached value is returned immediately with no network call.
• On a cache miss, the cache queries that specific peer using a whitelist option and stores the result.
• The cache includes configurable max size with LRU eviction, TTL-based expiry with background cleanup, and hit/miss stats for observability.

To support querying a specific peer on cache miss, the p2p request manager's pickAvailableStream was updated to properly respect whitelists:

• When a request carries a whitelist, only those streams are considered as candidates.
• The whitelist and blacklist data structures were also changed from plain maps to SafeMap for thread safety.

The integration touches three files in the staged sync pipeline:

• In stage_bodies.go, identifySyncedStreams now calls bnCache.GetBlockNumber() per stream instead of making a direct protocol request.
• In syncing.go, estimateCurrentNumber routes through bnCache.doGetCurrentNumberRequest() which queries any available stream with high priority and caches the responding stream's result.
• The cache itself is initialized with default configuration in staged_stream_sync.go and lives on the StagedStreamSync struct.

Comprehensive unit tests cover cache hits, misses, threshold logic, eviction, expiry, invalidation, and reset.

[mur-me]

Devnet is looking good, let's put it on the testnet