accounts/keystore: add Close method and improve finalizer documentation

[oooLowNeoNooo]

Addresses the TODO comment in KeyStore.init() regarding the finalizer not triggering properly due to references held by unlocked keys.

Problem:
The runtime.AddCleanup finalizer was not triggering because unlocked keys with
active timers maintain references to the KeyStore instance, preventing garbage
collection. This could lead to resource leaks where the account cache remains
open indefinitely.

Solution:

1. Added a Close() method to KeyStore that properly cleans up unlocked keys
   and stops their timers, allowing the finalizer to trigger correctly
2. Added a 'closed' flag to prevent double-close issues
3. Updated the TODO comment to explain the solution and recommend explicit
   Close() calls
4. Added comprehensive tests for the new Close() method