Ruby 2.1+ Performance Improvement, Threadsafe Cache Refactor and New Cache Methods

Hello,

@Seberius also I have given push access to you for this repo just in case you need it. (we should still do PRs for these bigger changes though)

Thanks!

As discussed in #3 , this is a general improvement branch to ease the implementation of a TTL cache.

First some notes about 1.8 support.

The only notable issue is that the benchmark (it was changed slightly to run in 1.9.3+) will no longer run in Ruby 1.8.7, as one of the development dependencies (one of guard's) is not available in 1.8.7. I can work more on this if wanted.
If the next release is going to be new feature release with a major version bump, a consideration may be to remove the legacy cache and drop pre-1.9.3 support, leaving 0.8.4 as the last 1.8 release. This branch and the next TTL branch do not improve the functionality or performance of the legacy implementation (other than the addition of #key?), so using it over 0.8.4 would not offer a benefit to a Ruby 1.8 user.

Now on to the branch features

Ruby 2.1+ Performance Improvement

Ruby 2.1 appears to have resolved the performance issue reported in https://bugs.ruby-lang.org/issues/8312. We can get some significant improvement by using #shift in Ruby 2.1+. Please refer to the changes made to README.md for benchmark results (both ran on the same machine).

Threadsafe Cache Refactor

The ThreadSafeCache implementation has been refactored as mixin, namely LruRedux::Util::SafeSync. The mixin will be compatible with the TTL cache, which cuts down on maintenance overhead.

New Cache Methods

A new cache method #key? and its alias #has_key? have been added. This is method is equivalent to Hash#key? and is common in other cache implementations (e.g. clojure.core.cache). This method was also added to a LruRedux fork by @AlexSc, but never pull requested back.

A new alias of #delete called #evict has been added. This is equivalent to the #evict method proposed in #3 and is common in other cache implementations (e.g. clojure.core.cache).

Tests have been updated to cover the new method.

Typos in changelog

FIxed a couple typos in the changelog.

Tests

This branch is currently passing all tests on 1.8.7-p375, 2.0.0-p643, 2.2.0, jruby-1.7.19 and jruby-9.0.0.0-pre1.

Fabulous run in 0.003189s, 7212.2923 runs/s, 21009.7209 assertions/s.

23 runs, 67 assertions, 0 failures, 0 errors, 0 skips

SamSaffron / lru_redux