fix memory leak in recently closed table

[jcs]

fac31e7 only pruned @streams_recently_closed after checking @oldest_stream_recently_closed which only ever gets set by the very same conditional.

On a busy HTTP-2 client (the same one that ran into #73), it will leak many hundreds of megabytes over a few days.

A simple reproducer:

$LOAD_PATH.unshift(File.dirname(__FILE__) + "/lib/")
require "http/2"

client = HTTP2::Client.new
client.on(:frame) {|bytes| }

100_000.times do
  client.new_stream.close
end

sleep 16
client.new_stream.close

puts "entries in @streams_recently_closed: " \
  "#{client.instance_variable_get(:@streams_recently_closed).size}"

Before this fix:

~/code/http-2$ ruby leak.rb                                       
entries in @streams_recently_closed: 100001

After:

~/code/http-2$ ruby leak.rb 
entries in @streams_recently_closed: 1

[HoneyryderChuck]

awesome find 👍