Improve ledger cache management with idle timeout

[bplatz]

Changes

Ledger Release

• Added fluree.db.api/release-ledger - Removes ledger from connection cache and cleans up resources (index queues, nameservice subscriptions)
• Release is idempotent and thread-safe (safe to call multiple times on same ledger)

Idle Timeout

• Added :ledger-cache-idle-minutes config option (default: disabled)
• Background loop checks for idle ledgers every minute and releases them
• Respects active indexing operations - resets idle timer if indexing is in progress
• Tracks last-accessed timestamp per ledger in connection state

Disconnect Improvements

• Added :disconnecting? flag to prevent new operations during disconnect
• Changed to release all cached ledgers in parallel using async/merge
• Logs warnings for release errors but continues with disconnect
• Stops idle cleanup loop before releasing ledgers

Implementation Details

• connection.cljc: Added start-idle-cleanup-loop, touch-ledger-access, close-ledger-resources, release-ledger
• api.cljc: Updated disconnect for parallel release, added release-ledger wrapper, updated validate-connection
• nameservice/storage.cljc: Helper functions for indexing status tracking

Test Coverage

Added test/fluree/db/connection/disconnect_test.clj with 4 tests (8 assertions):

• Disconnect prevents new operations after starting
• Disconnect releases multiple ledgers in parallel
• Idle cleanup loop is created when configured
• Release ledger is idempotent (race condition safety)