[udp] make response message cache configurable and update default

[hasheddan]

Modifies UDP conn implementation to allow for a custom response message cache to be provided. This change is implemented in such a way that API compatibility with previous v3 releases is preserved. Users may now provide an option to supply their own cache implementation. Structured messages, rather than their serialized representation, are now passed to the cache implementation to allow for caching decisions to be made in the cache implementation. For example, it may be desirable to skip caching blockwise message responses if the entire underlying data being transferred is also cached. The cache implementation is responsible for cloning messages or otherwise ensuring that it is not storing data that may subsequently be modified.

A slight functional change is also included in this update in that the default cache implementation now has pseudo-LRU functionality via ccache. When 30 newer messages are added to the cache, the 31st is marked for deletion. Note that deletion is not immediate in ccache and expired items can be returned. A check is added to ignore items that are returned expired.

Further performance improvements can be made in the default cache implementation. For example, while this implementation dramatically improves memory consumption for connections performing large volumes of requests, it also increases the total overhead of every new connection. However, because the implementation is now pluggable, improvements can be made without requiring updates to the library or breaking API changes.

Fixes #586

Note: keeping in draft for discussion at the moment.

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[hasheddan]

I have opened #590 as a replacement for this PR. I've played around with a number of popular Go in-memory caches, and though I do think the default cache used should employ some LRU functionality to keep the size bounded, all implementations I have tested have exhibited one or more of the following drawbacks:

• They don't also support TTL expiration.
• They are designed to run forever and do not expose functionality for stopping their background worker process (Why Expirable-LRU delete Close method？ hashicorp/golang-lru#159).
• There are questions regarding cleaning up the cache after use (Cache can't be garbage collected karlseguin/ccache#3, Stop can cause race detector errors in testing karlseguin/ccache#59, Memory leak during cleanup karlseguin/ccache#81).
• They require cleanup at all. Initially I liked the property of the cache expirations being divorced from the CheckExpirations() functionality of the udp.Conn. However, this mandates that Close() be called, or Run() be called and eventually exit. If a udp.Conn is created and then not manually cleaned up, resources could be leaked.

I think we could add minimal LRU functionality to the existing cache, though for now #590 makes it such that caching decisions can be owned by consumers, which allows for providing an LRU cache if desired. I'd prefer to move forward with that strategy and see if we can determine a better default implementation based on feedback.

[hasheddan]

Replaced by #590