Improve performance of set_expiration

[saverio-kantox]

Fixes #118 and #121

Array.bsearch is in ruby 2.0.

The difference with #121 is that here we manually build the index search using plain bsearch item on the array of (item, index) pairs, and then extract the index from it.

Before:

> bundle exec ruby performance.rb
setex
21.502102999947965
set
0.09983500000089407

After:

> bundle exec ruby performance.rb
setex
7.995712999952957
set
0.08754800003953278

Test file:

require 'mock_redis'
require 'benchmark'

VALS = Array.new(10_000) do |i|
  ["some_key:#{i}", 3000 + rand(10)]
end

mr = MockRedis.new

puts 'setex', (
  Benchmark.realtime do
    VALS.each do |key, exp|
      mr.setex(key, exp, 'some value')
    end
  end
)

puts 'set', (
  Benchmark.realtime do
    VALS.each do |key, exp|
      mr.set(key, 'some value')
    end
  end
)

[sds]

Thanks for the fix, @saverio-kantox.

Now that Ruby 2.2 is EOL, we'd be open to accepting a fix that uses bsearch_index if it faster. However, if the difference is negligible this will totally suffice!