Improve memory usage and client robustness

[tomas]

Potential memory leaks:

• Entity and Relation now hold a WeakRef to the client/strategy, so entity graphs retained by application code (caches, sessions) no longer prevent the Strategy, its Faraday connection pool, and auth credentials from being garbage collected. A clear RuntimeError is raised if a link is followed after the strategy has been GC'd.
• Configuration#logger now memoises with ||= instead of ||, preventing a fresh Logger from being allocated on every call when no logger is set.
• Strategy no longer allocates an empty Proc.new{} when no on_new_token block is given; on_new_token is nil and callers use &.call.

Code improvements:

• Oauth2Strategy#retryable: remove redundant begin/end wrapper.
• Replace default Faraday::HttpCache::MemoryStore (unbounded) with BoundedMemoryStore, which evicts the oldest entry once the cache reaches 500 entries, capping in-process memory growth in long-running servers.

New features:

• Client#close: nilifies the memoised Faraday connection so it can be GC'd.
• Client accepts :timeout and :open_timeout options and forwards them to Faraday, making request timeouts configurable without monkey-patching.
• TooManyRequestsError (HTTP 429) added to the error hierarchy so callers can rescue rate-limit responses distinctly from other transport