Add optional TTL-based cleanup for httpcache to reduce disk and memory usage

[wswsmao]

Problem

The current disk-backed HTTP chunk cache can consume significant storage over time:

• Cached chunk files may keep accumulating under the local httpcache directory.
• Long-running nodes can retain stale cache data much longer than needed.
• Existing behavior does not provide a built-in expiration mechanism for on-disk HTTP cache entries.

Solution

This PR introduces an optional TTL-based cleanup for disk-backed HTTP chunk cache entries:

• When http_cache_chunk_ttl_sec > 0 and the HTTP cache uses a disk-backed mode, a background janitor is started during resolver initialization.
• The janitor periodically scans the httpcache directory and removes expired cache files based on file mtime.
• The janitor skips the wip directory to avoid interfering with in-progress cache writes.
• The feature is disabled by default to preserve existing behavior.

Configuration

Add the following config option:

• http_cache_chunk_ttl_sec
  • Default: 0 (TTL cleanup disabled)
  • > 0: enable TTL cleanup with the given value in seconds (e.g. 120)

Example:

http_cache_chunk_ttl_sec = 120

[wswsmao]

@ktock