SSBC-EM2.3-CACHE1: Implement caching of full changeset specs

Created by: mrnugget

Part of https://github.com/sourcegraph/sourcegraph/issues/26142

• Extract createChangesetSpecs from src-cli into sourcegraph/lib and use in src-cli - 1d
  • https://github.com/sourcegraph/sourcegraph/pull/26997
  • https://github.com/sourcegraph/src-cli/pull/648
• Add implementation of ExecutionCache that writes JSON LogLines on write-events 0.5d
  • https://github.com/sourcegraph/sourcegraph/pull/27061
  • https://github.com/sourcegraph/src-cli/pull/649
• Make src batch exec use this cache 0.5d
• Server-side: extract cache-write events from log entries and save to new batch_spec_execution_cache_entries table with K/V layout 2d
  • key is cache key
  • value is text/jsonb/binary/whatever
  • version is a number that signifies format used in cached value
  • last_used_at
  • created_at
  • https://github.com/sourcegraph/sourcegraph/pull/27125
  • https://github.com/sourcegraph/sourcegraph/pull/27212
• Extract implementation of TaskCacheKey from src-cli to sourcegraph/lib 2d
• In workspace resolution: build cache key, check against database, set boolean cached_result_found on batch_spec_workspace, if cache hit: build changeset specs, persist, wire up correctly 1d
• In workspace resolver: if cached result found, return CachedResultFound: true and State: completed 0.5d
• When creating batch spec workspace execution jobs: skip workspaces with where cached_result_found boolean is set to true 0.5d
• UI: Show workspace.CachedResultFound 0.5d
• UI: All workspace.Steps are shown as skipped 0.5d
• Ensure that steps.env forwarding isn't usable in SSBC because it would break caching. 0.5d
• Periodic background job that cleans up old cache entries 1d
  • If possible: use table size (max X gigabyte) and evict LRU entries first
  • https://github.com/pulls?q=is%3Apr+author%3Amrnugget+archived%3Afalse+user%3Asourcegraph+is%3Aclosed
• In src batch exec: disable full-spec-caching in coordinator 0.5d
  • https://github.com/sourcegraph/src-cli/pull/653
  • Remove -clear-cache flag https://github.com/sourcegraph/sourcegraph/pull/27338
• Ensure that recreated-from-cache changeset specs are also cleaned up in replaceBatchSpecInput 0.5d