Improved performance of flush_cache and prime_cache when parameters are passed to them.

[jrafanie]

The more methods defined on the class and the more methods that are asked to flush, the slower the code becomes because Array Math is performed in a loop. The attached pull requests minimizes this.

From the original rails ticket from @Fryguy: https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/5517-activesupportmemoizable-flush_cache-is-very-slow4#ticket-5517-3:

"After more playing around with this, I realized the issue is not with calling methods + private_methods + protected_methods directly, but instead they are being called IN A LOOP of the parameters passed in. So, if you pass, for example, 20 method names to flush cache, it will call methods + private_methods + protected_methods 20 times. This is why I was seeing such horrible performance. flush_cache by itself wasn't the problem, it was flush_cache with parameters...the more the worse."

Before patch:

>> Benchmark.realtime { 100.times { v.unmemoize_all }}
=> 1.62846398353577
>> Benchmark.realtime { 100.times { v.flush_cache *MEMOIZED_METHODS }}
=> 40.7326259613037

After patch:

>> Benchmark.realtime { 100.times { v.unmemoize_all }}
=> 0.400160074234009
>> Benchmark.realtime { 100.times { v.flush_cache *MEMOIZED_METHODS }}
=> 0.0130050182342529

See this branch for a slow test example without this optimization: https://github.com/ManageIQ/memoist/tree/slow_test_without_optimization

ruby-1.9.3-p194-falcon@328 (slow_test_without_optimization) ~/Code/github/ManageIQ/memoist $ rake test                                             
/Users/joerafaniello/.rvm/rubies/ruby-1.9.3-p194-falcon/bin/ruby -I"lib:test" -I"/Users/joerafaniello/.rvm/gems/ruby-1.9.3-p194-falcon@global/gems/rake-0.9.2.2/lib" "/Users/joerafaniello/.rvm/gems/ruby-1.9.3-p194-falcon@global/gems/rake-0.9.2.2/lib/rake/rake_test_loader.rb" "test/memoist_test.rb" 
Run options: 

# Running tests:

....................

Finished tests in 0.623739s, 32.0647 tests/s, 131.4652 assertions/s.

20 tests, 82 assertions, 0 failures, 0 errors, 0 skips

And here's a branch with the same test and the optimization:
https://github.com/ManageIQ/memoist/tree/flush_and_prime_cache_optimization_with_slow_test_example

ruby-1.9.3-p194-falcon@328 (flush_and_prime_cache_optimization_with_slow_test_example) ~/Code/github/ManageIQ/memoist $ rake test                                                             
/Users/joerafaniello/.rvm/rubies/ruby-1.9.3-p194-falcon/bin/ruby -I"lib:test" -I"/Users/joerafaniello/.rvm/gems/ruby-1.9.3-p194-falcon@global/gems/rake-0.9.2.2/lib" "/Users/joerafaniello/.rvm/gems/ruby-1.9.3-p194-falcon@global/gems/rake-0.9.2.2/lib/rake/rake_test_loader.rb" "test/memoist_test.rb" 
Run options: 

# Running tests:

....................

Finished tests in 0.006318s, 3165.5587 tests/s, 12978.7908 assertions/s.

20 tests, 82 assertions, 0 failures, 0 errors, 0 skips

[matthewrudy]

Cool thanks Joe.
The tests all pass and the diff seems to make sense,
so I'll give it a merge!