Compressable#decompress Improve performance

[ksss]

This patch expect to reduce copy string object.

benchmark with minor revisions in https://github.com/fluent/fluentd-benchmark/tree/182f773eb2e2f0ba96520d80cd2a1a8541e05875/one_forward

agent.conf

<source>
  @type tail
  path dummy.log
  pos_file /var/tmp/_var_log_dummy.pos
  format none
  tag dummy
</source>
<match dummy>
  @type copy
  <store>
    @type forward
    flush_interval 0
    try_flush_interval 1
    buffer_chunk_limit 1m
    buffer_queue_limit 64
    num_threads 32
    compress gzip
    # phi_threshold 35
    <server>
      host 127.0.0.1 # FIX ME
      port 24224
    </server>
  </store>
</match>

receiver.conf

<source>
  @type forward
  port 24224
</source>
<match dummy>
  @type flowcounter_simple
  unit minute
</match>

dummer.conf

configure 'sample' do
  output "dummy.log"
  message "time:2013-11-25 00:23:52 +0900\tlevel:ERROR\tmethod:POST\turi:/api/v1/people\treqtime:3.1983877060667103"
end

$ bundle exec dummer -c dummer.conf -r 300000

before

2018-06-24 11:29:23 +0900 [info]: #0 plugin:out_flowcounter_simple	count:5423147	indicator:num	unit:minute
2018-06-24 11:30:23 +0900 [info]: #0 plugin:out_flowcounter_simple	count:5456601	indicator:num	unit:minute
2018-06-24 11:31:23 +0900 [info]: #0 plugin:out_flowcounter_simple	count:5406923	indicator:num	unit:minute
2018-06-24 11:32:23 +0900 [info]: #0 plugin:out_flowcounter_simple	count:5449504	indicator:num	unit:minute
2018-06-24 11:33:23 +0900 [info]: #0 plugin:out_flowcounter_simple	count:5381575	indicator:num	unit:minute

after

2018-06-24 11:37:55 +0900 [info]: #0 plugin:out_flowcounter_simple	count:5918921	indicator:num	unit:minute
2018-06-24 11:38:55 +0900 [info]: #0 plugin:out_flowcounter_simple	count:6396454	indicator:num	unit:minute
2018-06-24 11:39:55 +0900 [info]: #0 plugin:out_flowcounter_simple	count:6604293	indicator:num	unit:minute
2018-06-24 11:40:55 +0900 [info]: #0 plugin:out_flowcounter_simple	count:5642139	indicator:num	unit:minute
2018-06-24 11:41:55 +0900 [info]: #0 plugin:out_flowcounter_simple	count:6128793	indicator:num	unit:minute

(about 13% increase)

fluentd commit: 84c2194
dummer (0.4.0)
fluent-plugin-flowcounter-simple (0.0.4)

[repeatedly]

Good catch! Thanks!