Add cleanup block to FastCache::Cache, with RSpec tests.

[joelnordell]

This allows the user to specify a block to be called whenever an object is removed from the cache, to perform some cleanup action on it:

Example usage scenario -- caching connections that should be closed upon removal from the cache:

  cache = FastCache::Cache.new do |conn|
    conn.close
  end

  cache.fetch('/some/connection') do
    Connection.new
  end

[ssimeonov]

Please provide the block as an &param. This type of defaulting is one of the obscure features of Ruby that saves a few characters at the expense of readability.

[ssimeonov]

A high level question about this PR: does it make sense to add the cleanup behavior to a simple fast cache? If the use case is to cache connections, of which there typically are not many, and which do slow I/O, does it matter whether the cache is fast or slow?

[joelnordell]

Good comments -- I'll push another commit that addresses them.

Regarding the high level question: my use case is actually not caching slow things like connections, but caching allocated data structures for a native library (via FFI) which contain open file descriptors that need to be manually cleared/closed.

I'm using FastCache not only for its speed, but also for its convenient API and the ability to specify both a maximum size and a time-to-live.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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